The Role of Vitrification in Spanish Reproductive Labs: A Cryo-revolution Led by Strategic Freezing

Assisted reproductive technologies have expanded vastly and are frequently addressed using the language of revolution. The last two decades witnessed important transformations in Spanish repromarkets, some of which are linked to one freezing technique: vitrification. This cryotechnique tends to be presented by professionals as revolutionary as well, even if in a humbler way: a technique with the capacity to revolutionize reproductive labs. Celebrated for its ability to freeze oocytes, the introduction and assimilation of vitrification have implied many transformations in Spanish clinics and the broader reproductive industry in Spain. This paper describes the role of vitrification from the point of view of the labs, drawing on interviews with professionals and observations of laboratory work, and focusing on the changes, which include but are not restricted to freezing oocytes. I argue that vitrification has increased the role of cryopreservation in reproductive labs overall in Spain, expanding the use of strategic, short-term freezing of embryos. This is discussed as part of a “cryomedicalized” turn, using Charlotte Kroløkke and Anna Sofie Bach’s term, which enlarges the scope of medicalization through optimization and prevention, in the context of late reproduction and a strong reproductive market.


Introduction
Since the first baby was born after in vitro fertilization (IVF) in 1978, assisted reproductive technologies (ARTs) have expanded globally, giving rise to different "biomedical modes of reproduction" (Thomson 2005) and reproductive markets. The changes IVF brought with it have received a great deal of attention from social studies (e.g., Franklin 1997Franklin , 2013Almeling 2011;Waldby 2019). IVF has in itself become more than a reproductive technology, because it provides "a technological platform through which reproductive substance could be both seen and handled," becoming "a major global platform for the health sector and emergent bio-industries" (Franklin 2013, 246, 42). This platform paved the way for other developments such as embryonic genetic diagnosis, gestational surrogacy, human stem cell research on embryos, and cryopreservation of embryos and oocytes (Franklin 2013;Inhorn 2020).
The application of cryopreservation to embryos and oocytes, one of the practices enabled by IVF, profoundly changed reproductive timing, allowing for storage of reproductive material for undetermined periods. The main technique used for freezing until well into the 2000s was slow freezing, where embryos were routinely frozen and stored for later use, either in subsequent IVF cycles when the first failed to achieve a pregnancy or at an undetermined point in the future. It was not until the introduction of fast freezing or vitrification that oocytes could be frozen with similar results. For this reason, cryopreservation within ARTs first received attention mostly in relation to embryos. Attention came from bioethics and legal studies, with a focus on disputes over frozen embryo use or property (e.g., Amorós 2007;Dolgin 1994), or on questions raised by the storage of biological material thought to be between "the sacred and the profane" (Thompson 2005;Jasanoff and Metzler 2020). Since the freezing of oocytes became possible, a more Science and Technology Studies-oriented approach has studied its associated practices-mainly the freezing for a person's own use and the emergence of egg banking within so-called egg donation programs (e.g. Baldwin 2017; Waldby 2019; Van de Wiel 2020).
Technologies of cold have greatly expanded in the last century, with some authors critically examining how "[t]he ability to arrest biological processes in order to reanimate them at some point in the future has profoundly transformed the terms of life" (Lemke 2021, 2). Among these approaches, the term "cryopolitics" has been suggested because it "turns our attention to what happens when temperature is used to reorient life" (Radin and Kowal 2017, 12). Lemke (2019Lemke ( , 2021 argues that we are witnessing a new "politics of suspension." These conceptualizations are useful to make sense of storage practices used in cryopreservation. Charlotte Kroløkke and Anna Sofie Bach, following studies of cryopolitics and biomedicalization (Clarke et al. 2003), propose the term "cryomedicalization" to describe how technologies of cold are deepening the medicalization and optimization of female aging bodies (Kroløkke and Bach 2020). In this paper, I use cryomedicalization to conceptualize how "suspending" embryos in Spanish clinics is going beyond logics of storage and deepening those of medicalization through "strategic freezing." Reproduction has for decades been characterized by increasing medicalization, which brings with it heavily gendered ideals that impact how women think about their bodies and reproductive processes, their hopes and fears around them, and their own subjectivities (Blázquez Rodríguez 2009). The introduction and expansion of ARTs intensified this medicalization, lately moving into the medicalization of fertility itself (Van de Wiel 2020), made possible through the introduction of vitrification, which can freeze oocytes with good enough success rates for the treatment to be commercialized (Van de Wiel 2020; Waldby 2019). Medicalization in itself is so generalized that Nikolas Rose suggests taking it as "the starting point of an analysis, a sign of the need for an analysis," although it "should not be the conclusion of an analysis" (Rose 2007b, 701-2). This paper takes the medicalization of reproduction within ARTs as a sign of the need for an analysis. It investigates how practices linked to vitrification in Spain were both enabled by the logics of optimization and prevention, which facilitates the subsequent process of cryomedicalization (Kroløkke and Bach 2020).
Spain is one of the main reproductive hubs in Europe. Its fertility industry has specialized in dealing with problems associated with reproducing later in life, and in treating international patients, two specializations connected to the expansion of treatments with donated eggs (Molas 2021). Dealing with oocytes might be related to the rapid success and assimilation of vitrification in the country, making it an interesting case study.
This paper documents how the widespread implementation of vitrification affects Spanish reproductive labs and industry. First, I introduce methods and materials, the Spanish context, and the role of cryopreservation within ARTs. Then, I move to the findings, examining the role of vitrification as a whole and the role of strategic freezing as a particular type of embryo vitrification. I convey professionals' views that vitrification was a "revolution" for their reproductive labs, as cryotechnologies moved from the periphery of IVF to the center, thanks to three advances: being able to freeze oocytes, increased success rates of embryo freezing, and the introduction of a very flexible technique. Within the findings, oocyte freezing is reflected upon and the relevant literature is presented. To better focus on the knowledge gap, I examine the less studied changes that embryo vitrification implied for these reproductive clinics.
Even though embryos were routinely frozen with slow freezing techniques, the higher efficiency of vitrification for freezing and thawing embryos-and the flexibility of the technique itself-is linked to the expanding role of "strategic freezing." With slow freezing, embryo freezing had been mainly used for storage purposes, but "strategic freezing" points to freezing as a strategy to optimize a given IVF cycle by deferring embryo transference, that is, to extend the time between fertilization and transference to introduce a new time window in which particular actions can take place. It is precisely in this newly created time that cryomedicalization takes place, through newly enabled ways of dealing with risk or organizational matters (such as schedules optimized for reproductive travelers or the ability to set a specific date for embryo transfer to take place). In these cases, freezing is used to broaden the range of strategies that can be adopted within IVF treatments-strategies that deepen medicalization by further enabling logics of prevention and optimization within them.

Methods and Materials: Studying Vitrification in Spain
This paper draws mainly on qualitative data I collected in Spain, although it also uses secondary data derived from the Spanish Fertility Society annual reports (SEF 2009-19) to contrast qualitative with quantitative information. The qualitative data consist of observation of laboratory and medical routines in an IVF clinic (eight weeks), divided into two periods between 2020 and 2021, and interviews with 22 professionals from this and five other clinics, including biologists, doctors, nurses, and one psychologist, which took place between 2019 and 2021 in Madrid and Barcelona. As the private sector is the most important one in Spain, most interviews and observations took place within it. Nonetheless, one professional worked in public schemes and at least three others had worked in the public sector in the past. Their insights reinforced the idea that focusing on the private sector was the better choice to study vitrification in Spain. The public sector has more restricted access to new technologies and has limited influence on the overall shape of assisted reproduction in the country (Pérez Milán 2011). Most interviews took place in person, although some were conducted online after the outbreak of the COVID-19 pandemic. This also affected the observations, which were suspended in 2020 and resumed in 2021.
Field notes and interviews were mainly in Spanish. All the transcribed material was later analyzed with the help of a qualitative software program, and coded (in English). Quotes in this article were translated by me and corrected by a professional editor. Pseudonyms were given to all the interviewees quoted to comply with the anonymity agreement approved by the Ethics Committee of Goethe Universität Frankfurt and signed by both parties.

Spanish Reproductive Clinics: A Fertile Environment for the Blossoming of Vitrification
Spain is a paradigmatic example of both the decline in fertility and the expansion of the reproductive industry. It has one of the lowest fertility rates in Europe and one of the largest numbers of clinics in relation to the population (Wyns et al. 2021). This coexists with what some authors call "structural infertility" (Alvarez and Marre 2021), and which I treat here as part of a broader reproductive crisis. The idea of a crisis points to a combination of increasing precariousness and inequality affecting the younger generations, and a widening gap between reproductive desires and realities, which is to say that people say they want to have more children and earlier than people in Spain currently do (Castro et al. 2020;INE 2018;Lafuente Funes 2021). This reproductive crisis is linked to high youth unemployment rates (40 percent), low fertility rates (1.24), and a high average maternal age for the first child (over 31;INE 2018). Even though some of these trends are present in other countries, the situation might be more aggravated in Spain where shifts have been drastic since the end of the dictatorship in the late 1970s (Alvarez and Marre 2021). In a context where public measures to secure care for children or to enhance earlier reproduction are insufficient (Ibarra Roca et al. 2021), the fertility industry has bloomed, specializing in offering solutions to problems linked to reproducing later in life. In that sense, it offers individualized and medicalized solutions to deal with broader social problems entangled in postponing reproduction. This context might be related to the rapid assimilation of vitrification in the country, given it offers ways to deal with late reproduction.
The Spanish reproductive market relies on flexible regulation, which enables reproductive tourism and egg donation treatments (Kroløkke 2014;Rivas and Á lvarez 2021;Molas 2021). Clinics provide varied services to fee-paying clients, particularly in relation to egg donation and preimplantation genetic testing (PGT; Pavone and Lafuente 2017;Perler 2021). Egg providers' reproductive capacity (transferred through their gametes) is crucial in attracting patients from other countries, and thus the two aspects are mutually reinforcing and shape the market in one direction that makes them highly dependent on the availability of oocytes (Lafuente-Funes 2020). Indeed, around 50 percent of treatments with donated oocytes in Europe take place in Spain, and most nonresidents seeking treatment go there to acquire third-party oocytes (Shenfield et al. 2017;Wyns et al. 2021).
The fact that this market is so focused on oocytes (with an important presence of oocytes donated anonymously by third-party, young people) might have encouraged clinics to quickly introduce vitrification, a technique that is much better suited to freezing oocytes. Spanish clinics were among the first in the world to use vitrification within egg donation (Cobo et al. 2008(Cobo et al. , 2015Solé et al. 2013;Solé Inarejos 2014). Spain was found to be the main user of oocyte cryopreservation in Europe by the ESHRE Working Group on Oocyte Cryopreservation, performing 61.6 percent of all cycles in Europe by 2013 (Shenfield et al. 2017).
For all these reasons, looking at Spain is key to further understanding the potential of vitrification, as well as the promises and the logics it might be pushing further.

From Slow Freezing to Vitrification: Cryopreservation within IVF
Although it took more than 20 years of development, vitrification has transformed the IVF industry. (Schiewe and Anderson 2017) Cryopreservation was important for ARTs from a very early stage (Cohen et al. 2005). Alan Trounson's team in Australia pushed it at the beginning: after they announced the first pregnancy achieved from a frozen embryo in 1983 (Trounson and Mohr 1983), in 1984, their successful use of slow freezing techniques led to the birth of the first baby whose conception was assisted through these techniques. Already in the 1980s, this team was doing research on fast(er) freezing protocols (Trounson, Peura, and Kirby 1987), as a way of avoiding the formation of ice crystals that complicated successful thawing, particularly in the case of oocytes. Fast freezing was difficult to employ in clinical use, and because slow freezing had good enough results with sperm and embryos, it started to be widely used, mainly for banking and storing. Sperm banks emerged and expanded, mainly for third-party reproduction. Freezing embryos also became a routinized practice whenever an IVF cycle resulted in more embryos than needed to be transferreda common case because several oocytes tend to be extracted for fertilization in each cycle, following the administration of hormone treatments. Across the countries and years, the tendency to implant fewer embryos resulted in storing more of them, which frequently caused storage problems for clinics. Storing practices are regulated differently in each country, with time limits in some, and different routes for thawing and discarding them in others. The practice of freezing, storing, and discarding embryos opened up legal discussions over who owned them (particularly in cases of divorce and decease) and their status (e.g., Amorós 2007;Dolgin 1994;Thompson 2005; Jasanoff and Metzler 2020), although it quickly became a standardized practice. Spain is again an example of flexible regulation in these matters, having no time limit for keeping embryos or oocytes frozen, although it has specific regulations making it rather difficult both for patients and clinics to discard these cells (Benavente and Farnós 2015), except in cases where they are considered potentially nonhealthy through PGT, as I discuss below.
As a result of the low success rates of oocyte survival with slow freezing techniques, oocyte freezing was mostly restricted to the preservation of oocytes in medical cases (i.e., for women undergoing medical treatments that affected their ovarian reserve, such as chemotherapy). In these cases, oocytes were frozen, although it was accepted that there would be a high failure rate. It was not until 2012 that the fast freezing technique of vitrification stopped being regarded as experimental in the case of oocytes in the United States and Europe (ASRM 2013;ESHRE 2012). Vitrification, first celebrated for its ability to freeze oocytes, was already being used at that time in many Spanish clinics, which had published good results of vitrification (Cobo et al. 2008;Solé et al. 2013). The impact of freezing oocytes had been huge in the Spanish context, yet the introduction of vitrification had a wider effect for these clinics which remains understudied as I show.

Vitrification as a Revolution of the Labs: Professional Views and Main Transformations of Laboratory Work
I present here the views of professionals working at IVF clinics. They tend to emphasize the transformative role of vitrification, even describing it as having revolutionized their practice: Vitrification, so that you get the idea, has meant now, today, the greatest revolution in assisted reproduction for years and years. (Borja, biologist, February 2020) I would say that, with vitrification, everything has actually changed overnight, and for the best. Well, in fact, it has made possible the implantation of a few techniques that we could not even think about before. (Victoria, biologist, October 2019) So now with vitrification, even though slow freezing worked pretty well for us in embryos, vitrification has brought about a revolution. (Diana, medical doctor, November 2021) The emphasis on this technique being "revolutionary" struck me immediately. It is not uncommon language in the area of IVF, where hype is frequent (see Franklin 2008Franklin , 2013Inhorn 2017). Nonetheless, given that slow freezing was already a finely tuned technique for freezing embryos and sperm, why do these professionals see vitrification as such a game changer?
Mario, the biologist in charge of introducing this technique in a major Spanish clinic, explained that some clinics stopped using slow freezing with both oocytes and embryos after a while: Up until 2012, we used to do both vitrification and slow freezing, depending on the case. From 2007 on, we were already vitrifying oocytes as it was the only way to freeze them with some guarantee . . . and then from 2012, as the results we were getting were really good, then we also moved to do it with embryos. At the general level, I would say that from 2010 all labs started cryopreserving oocytes, and then also embryos with this technique. (Mario, biologist, October 2019) Even if only oocyte vitrification stopped being considered experimental around 2012, it was already in use in many Spanish clinics before that, as the quote reflects. This introduction implied a major shift: what we are talking about is that 20 percent of the treatments within a lab used to use frozen material, and now we are talking about almost 90 percent of everything that is done in in vitro fertilization going through freezing. So, both the volume of work within cryobiology labs and their importance have increased tremendously over the last 10 years. (Mario, biologist, October 2019) Cryopreservation gained importance for these labs, moving from a rather peripheral to a central role thanks to vitrification. In some cases, as described by Mario, this meant most IVF treatments go through cryolabs at some point. This increased presence of cryopreservation needs to be studied in the context of the broader medicalization of reproduction introduced through ARTs.

Freezing Oocytes: Social Freezing and the Emergence of Egg Banks
First, and as already reflected by many studies, vitrification implied the possibility of freezing oocytes with much better results (e.g. Cobo et al. 2008;Schiewe and Anderson 2017). As one biologist stated: At the beginning, when freezing oocytes by slow freezing we really did not know how the cycle would turn out. Expectations were quite low, both in egg survival and in fertilization and embryo development. Now with vitrification it works wonderfully. (Gina, biologist, March 2020) Vitrification opened up, among other options, the possibility of freezing and storing oocytes for a person's own use. These treatments to preserve eggs, randomly used and only for medical conditions with slow freezing, rapidly moved forward as "social" or "elective" freezing treatments. That is, the possibility of cryopreserving oocytes for later use by women that did not face health risks. Indeed, it was mainly oncological preservation at the beginning, but suddenly this started changing, and where we had 80 percent of oncological patients and 20 percent of elective preservation, things have turned around, and in our experience at least, [we have] 80 percent elective preservation, 20 percent oncological. That's enormous. (Diana, medical doctor, November 2021) Elective preservation, as this doctor calls it, involves the promise of rearranging reproductive times through vitrifying oocytes for an undetermined period. Baldwin (2017) was among the first scholars who studied women's rationalities when freezing their eggs in the United Kingdom. Other studies illuminated the ways in which women approach this treatment in the United States, Israel, and Turkey (Inhorn et al. 2022;Göçmen and Kılıç 2018). Interestingly, Van de Wiel (2020) looked at how these treatments are part of a particular politics of aging, considering both the role of planning and anticipatory logics at the subjective level, and the commercial logics of the "reproductive-industrial complex" it represents. All these studies show how women's reproductive trajectories are planned around freezing their eggs-around pausing of their "biological clock." Of course, this does not mean that these rearrangements are successful or straightforward, but rather that egg freezing "becomes an entry point into a long-term, technologically managed reproductive trajectory across the life course" (Van de Wiel 2020, 110).
The ability to suspend oocyte development also makes it possible to store and distribute oocytes in new ways, rearranging the time and place in which so-called egg donation treatments can occur, enabling further commercialization through egg banks-commercially storing donated eggs for third-party use in the future or in other locations.
With vitrification what was optimized was the freezing and thawing technique, and therefore you could, for example, freeze eggs from donors and make an egg bank, and [freeze] eggs from patients that wanted to freeze for their own use. (Chloe, biologist, September 2020) Catherine Waldby has closely studied the effect of this option of "making an egg bank" to which Chloe refers. Indeed, she explains how "frozen oocytes allow economies of scale and time management to be introduced in clinical practice" because "egg freezing enables the corporatization of egg donation" (Waldby 2019, 120, 125). The possibilities that vitrification and banks open up for rearranging commercial practices linked to donated oocytes and their distribution are enormous. This mainly affects the number of patients that can be treated after each donation cycle (as oocytes can now be divided and stored in egg banks), and how matching processes take place (as now they can search for a combination in much broader databases). In relation to the former, it has been suggested that the possibility of cryopreserving and batching oocytes might work as an incentive to hyperstimulate donors (Tober and Pavone 2019). The introduction of egg banking allows streamlining logics within (donated) oocyte management, making it possible to "exploit the divisibility of frozen oocytes" (Waldby 2019, 131). Furthermore, it may be possible for oocytes frozen for a person's own use in fertility preservation treatments to end up in donation circuits, if the women that freeze them in the first place do not want them for themselves, as Alexander Friedrich suggests in his work. In Spain, this can happen especially for women younger than 35 at the time of freezing, because under the current legislation oocytes of this age could be donated to other women-with consent from the patient. Also, new ways of valorizing oocytes may emerge that are different from those originally envisaged, such as within research, which Alexander Friedrich has described as a potential "secondary utilization of female fertility resources" (Friedrich 2020, 1).

Taking Embryo Survival (Almost) for Granted
As seen above, both biomedical and STS literatures have reflected on the possibilities opened up by the freezing of oocytes thanks to vitrification. Even if many scientific publications show that vitrification has also led to increasing success rates of embryo freezing (e.g., Rezazadeh et al. 2009;Schiewe and Anderson 2017), social science research has not yet analyzed how this has affected actual clinical practices in the context of reproductive medicine. Nonetheless, when asked what vitrification meant for their work, professionals often reflected on how it impacts embryo freezing: [with vitrification] we defer transfers with complete calm, I mean, the result is exactly the same as if you were doing the transfer fresh. (Diana, medical Dr., November 2021) In regard to embryos we are reaching survival rates close to 100 percent, that is 98 percent, and then pregnancy rates-that is, the viability of these thawed embryos-is as if they had not been frozen. (Mario, biologist, 2021) Even though embryos were routinely frozen with slow freezing techniques, vitrification greatly changed the expectations professionals have of them after thawing: Vitrification has made things much better, and the expectations we have toward an embryo thawing cycle for example have completely changed. Now we can count on the embryo that has been vitrified, but with slow freezing it was not so clear. The survival rate was lower. (Gina, biologist, March 2020) It affects positively as statistics change totally. I mean, the pregnancy rate and the children born alive rate increases when we start with vitrification. The survival rate is way higher than the one we had with slow freezing. With slow we were around 65 percent. With vitri you are at around 80 percent in oocytes and way over 90 percent in embryos. (Montse, biologist, November 2021) Professionals can now "count on" success, rather than feeling that the result is less certain. Not only are embryos expected to survive, but professionals expect freezing and thawing to have no effect on their later reproductive capacities whatsoever. Embryo freezing is seen as a much more effective and reliable technique than slow freezing. They do not take survival for granted-they know there is still a possibility of failure. Nonetheless, they explain a shift in the trust they have in embryos surviving freezing and thawing without affecting their later development as something that has modified how they relate to freezing altogether.

Not Only a Fast but Also an Adaptable Technique
Vitrification is at times also referred to as fast freezing. Once in the lab, one quickly understands the focus on speed in naming these techniques: cryobiologists introduce the cryocarriers with oocytes or embryos directly into liquid nitrogen in a super-fast, bare-hand move that ends up when the vials are correctly introduced into the bigger tanks of the cryostorage rooms. Indeed, watching their work routines and inquiring about how these have changed over the years made it clear that vitrification is introducing new entities (oocytes) into cryolabs, as well as increasing the success rate of entities (embryos) frozen before the advent of the technique. Vitrification is also changing the manner in which these cells are frozen, which has important practical dimensions too. Indeed, very high success rates are key to the widespread implementation of vitrification, as well as the flexibility of the routine when compared to slow freezing. Slow freezing is easy handling: supports are introduced into a machine that has slowly lowered temperature on the basis we have programmed it to do for two hours. So, whenever we started our shift we calculated, you organized yourself (ICSI and so on) around that. (Gina, biologist, March 2020) Similarly to Gina, several professionals explained that slow freezing took hours and was machine-dependent, making lab work rigid and requiring it to be organized around freezing times-even though the technique itself was very easy to learn and reproduce. By contrast, vitrification becomes very manual indeed, very manual or person-dependent. Depending on the formation of each embryologist. (Montse, biologist, November 2021) it must be really fast and it has a certain difficulty. Not everyone can do it and it also has a learning curve that affects the results. (Gina, biologist, March 2020) Vitrification takes a long time to learn and is heavily dependent on the biologist's ability to handle the cells very quickly and steadily and immerse them in liquid nitrogen.
But then, with vitrification . . . . First, at the lab level it was wonderful: because it makes a big difference whether it takes three hours to freeze or fifteen minutes. ( . . . ) And of course with much, much better results with vitrification. Much, much better. (Marisa, biologist, October 2021) This means that even if vitrification requires the acquisition of certain skills, it then becomes a flexible technique with very good results that, once learned, is easy to adapt to laboratory routines.
As we have seen, many professionals see vitrification as revolutionary for myriad reasons. Oocyte freezing is clearly one of these-but does not account for the vastly expanded role of cryopreservation in Spanish IVF labs. I argue that professionals' increased trust in embryo survival after vitrification, in addition to the technique's adaptability, is key to the growing role of freezing in these clinics.

Strategic Freezing and the Expansion of "Cryomedicalization"
Being able to increase efficiency so much in embryo and oocyte freezing has meant a shift in our strategy . . . A great range of strategies open up, both for in vitro fertilization with patients ready to get pregnant, and for those who just want to preserve their material. (Mario, biologist, October 2019) New strategies to deal with reproduction and fertility emerged in Spanish clinics with the popularization of vitrification from the mid-2010s, as Mario explains. During my research, I looked in IVF labs for strategic shifts linked to vitrification, finding that many are linked to embryo freezing for short periods of time. I describe this as strategic freezing, different to the wellestablished practice of freezing nontransferred embryos to store them (for medium-or long-term periods), which was common with slow freezing and continues to be used with vitrification.
In theory, vitrification should have almost no impact on final results. That's why we started with the idea of segmented cycles, where you obtain oocytes in a first step, you fertilize them . . . [then] you vitrify embryos, which are transferred to a second substitute cycle. (Carles, medical doctor, October 2019) In regular IVF cycles, oocytes are fertilized and, after three to six days of development, the ones that keep on dividing and look most suitable are transferred to the uterus. With these "segmented" or "substitute" cycles, embryologists open a window of time between fertilization and transference. In this newly created time frame, action can be taken in the form of strategies to prevent and manage "reproductive risks" or to optimize the organization of clinical timings and adapt them to the clinics' or clients' needs-for example, to wait until an international couple or a woman in treatment with donated eggs reaches the clinic. Through such strategic freezing embryos are "suspended" to either gain information about their characteristics (to decide whether to transfer or discard them) or to allow better preparation for the transference (allowing for a better endometrial preparation or organizing reproductive travel). Indeed, by suspending biological development of embryos, other processes are given more time to develop (e.g., the body to which the embryo will be transferred is allowed to recover from hormone treatment) or professionals are given time to acquire more information and evaluate what to do with the embryos (e.g., by conducting genetic testing). Overall, the period between an embryo is fertilized and it is transferred is prolonged so that the IVF process can be optimized.
From the three general characteristics introduced by vitrification and discussed above (freezing oocytes, increasing success rates for embryo freezing, and enabling a more flexible lab routine), I contend here that the second two translated into an increased use of strategic freezing in Spanish IVF labs. Even though short-term freezing could also be done with slow freezing, vitrification increased enormously in the clinics studied due to a combination of trust in embryo survival and the adaptability of freezing protocols enabled by vitrification.
In the last section, I document three types of strategic freezing widely used by IVF clinics in Spain: freeze-all treatments, decreased numbers of embryos being transferred per cycle, and increased use of PGT.

The Expansion of Freeze-all Cycles and Decrease of Embryos per Transference
A clear example of "strategic freezing" would be freeze-all cycles. That is, cycles in which no transference takes place after fertilization because all embryos are frozen. The Spanish Registry of IVF cycles did not collect data about freeze-all treatments from 2015 on. Since then, freeze-all treatments have steadily increased, with a total of 9,492 in 2015 and 16,984 in 2019 (SEF 2015(SEF -2019. Freeze-all treatments are frequently part of strategies to avoid risk: [in case of hyperstimulation] you freeze all: you just do not do the transfer and you avoid 100 percent the patient developing ovarian hyperstimulation syndrome. That is the only cause that has led to a patient dying as a result of assisted reproductive techniques. And the other major complication? Multiple pregnancy. In IVF it has always been around 25 or 30 percent of all pregnancies ( . . . ) now the doctors, the gynecologists, have come to trustit has not been easy but they have-that the material we are freezing has the same possibilities and they are not afraid of losing the chance of having transfers just because they are frozen. Therefore, they are less afraid of doing transfers with fewer embryos, they are no longer afraid of solo transfers. (Mario, biologist, October 2019) The risks most frequently mentioned in connection with assisted reproduction are multiple pregnancy and ovarian hyperstimulation syndrome (OHSS). The quote above presents vitrification as helping to address both these risks. On the one hand, OHSS is linked to hormone treatments used to promote ovulation of more oocytes in each cycle, but excessive stimulation can lead to severe problems that may be worsened by pregnancy. In these cases, deferring embryo transference in a "segmented cycle" lowers the risk of OHSS. Also, trust in successful embryo thawing helps professionals come to terms with reducing the number of embryos to be transferred per cycle, thus reducing the risk of multiple pregnancies. They no longer believe transferring one and freezing the others will vastly reduce the chances of achieving a pregnancy, because chances of pregnancy occurring with thawed embryos are similar to fresh ones. Of course, the tendency to implant fewer embryos cannot be solely attributed to vitrification, which is one among many processes contributing to a higher level of trust in embryo implantation, including, for example, time-lapse technology for monitoring embryo development. Nonetheless, as the biologist quoted above concludes, vitrification is key to making professionals "no longer afraid of solo transfers." The shifts enabled through vitrification have allowed professionals to minimize risks of multiple pregnancies and OHSS through strategic freezing, which opens a window of time between fertilization and transference.

Increased Resources for Embryo Screening Practices
Avoiding risk for patients is not the only way in which vitrification is used to optimize risk management: with vitrification in particular and cryopreservation in general, but specifically with vitrification as it is so efficient, it has been possible to open up new possibilities for treatment. For instance: we have the possibility of analyzing embryos genetically-as you need to freeze them while you wait for the result, and that takes a few days. (Victoria, biologist, October 2019) Even if PGT can take place without vitrification, it is much more efficient with it (due to its higher success rates). The increase in PGT use in Spain after the assimilation of vitrification is clear: if in 2014 the number of embryo transfers after PGT was 2,346, by 2019 this had increased to 7,685 (SEF 2014(SEF -2019. According to the professionals I interviewed, the increased use of PGT involves a mix of elements. First, and thanks to the optimization of embryo culture and the introduction of time-lapse technology, 1 embryos are increasingly being transferred at the blastocyst stage (after approximately six days of development rather than three, as was the norm before). The embryo has time to develop more cells, so the risk of taking a few to test them is lower. Second, and as the quote reflects, higher success rates of embryo freezing and thawing with vitrification make it possible to carry out PGT in more cases, because they can wait for results while the embryos remain vitrified. Third, and as a result, the framing of risk in these clinics has shifted the parameters within which preventative action should be taken. For example, reproducing later in life (late thirties and forties) is now considered enough of a risk factor to advise women to undertake genetic screening of their embryos: if you are old, or you consider yourself to be old, or doctors think you are old (laughs) and you say "well, I want to do a genetic test of my embryos; I do not have any condition but I do not want any kind of problem, I want an embryo that implants and it's healthy and so on"-you just do it. (Borja, biologist, February 2020) Indeed, "advanced maternal age" is one of the reasons for conducting PGT 2 recorded in the reports from the Spanish Fertility Association 3 : in 2009, it represented less than 27 percent, and 10 years later, it exceeded 58 percent (SEF 2009(SEF -2019. In these testing practices, some cells are extracted and analyzed for mutations. Even if most mutations are incompatible with life (meaning the embryo will not lead to a pregnancy or will produce a miscarriage), some mutations are not, such as genetic markers linked to Down's, Turner's, or Klinefelter's syndromes. These testing practices are part of selective reproductive technologies "used to prevent or allow the birth of certain kinds of children" (Gammeltoft and Wahlberg 2014, 201), which focus on selecting out rather than choosing for or editing to optimize (Wahlberg 2020). The optimization of cryopreservation brought by vitrification is linked to increased rate of embryo testing, which is carried out to suspend their development until results are ready to make an "informed choice" on which ones to transfer and which to discard-a choice that is already informed by a certain way of understanding, and desiring, healthy babies.

Optimization and Cryomedicalization: Understanding the Role of "Strategic Freezing"
The concept of cryomedicalization highlights "the entanglements between treatment, prevention, optimization, and rejuvenation" in ovarian tissue preservation (Kroløkke and Bach 2020, 301). In considering cryomedicalization, we can improve understanding of strategic embryo freezing in general, and the growing use of PGT in particular.
We do not choose in relation to phenotypical characteristics or things like that . . . . You are simply screening for disease. It is like if you were doing preventive medicine in a sense. You are avoiding the situation in which a woman has to face the dilemma of having or not having an abortion given she has an affected pregnancy, so it is like preventing this situation. (Montse, biologist, November 2021) To understand PGT as a form of "preventative medicine," as above, is indicative of the selective rationale that lies behind the routinization of screening and discarding practices in Spain. This rationale fits all too well with logics of cryomedicalization: if medicalization once focused on control and biomedicalization focused on transformation (Clarke et al. 2003), cryomedicalization points to how medicalized turns enabled through freezing are linked to optimization and prevention (Kroløkke and Bach 2020). Optimization occurs when "technologies of life are no longer constrained, if they ever were, by the poles of health and illness. These poles remain, but in addition, many interventions seek to act in the present in order to secure the best possible future for those who are their subjects" (Rose 2007, 18). In the case of strategic freezing, vitrification "suspends" embryo development to act in the present (enabling labs to come up with screening results, aspiring mothers to reach the clinic if they are coming from abroad, bodies to achieve a favorable endometrial environment, etc.) with a view to then "restarting" these embryos' development to secure an optimized IVF treatment, pregnancy, or end result-with the hope that it will enable, if not the best at least a better possible future. In that sense, strategic freezing in Spanish clinics is a clear example of the expansion of cryomedicalization (Kroløkke and Bach 2020).
The perspective of cryomedicalization reveals that cryotechnologies are being used beyond the logic of storage. Both optimization and prevention are key to understanding how IVF is allowing further medicalization thanks to the time window created by these short-term, strategic suspensions of embryo development eased by vitrification. The term also points to the role of cryotechnologies in further medicalizing female reproductive aging, for example, through the increased use of PGT.
In strategic freezing, embryos are not frozen for indefinite periods, but to create a specific short-term window to optimize a particular IVF project-which in Spain has already been defined as the search for healthy babies (Perler 2021). Cryomedicalized responses to this search for healthier offspring at later maternal ages imply a certain routinization of short-term embryo freezing that works by allowing reproductive clinics more space to improve their ability to optimize IVF in itself and selective practices within it, thus intensifying the medicalization of reproduction.

Concluding Remarks
The introduction and widespread adoption of vitrification have notable effects on the Spanish reproductive industry. In this paper, I set out to analyze this freezing technique, often described by IVF clinic professionals as revolutionary, and I have shown that it has a clear role in ordering today's landscape of reproductive medicine and markets in Spain. Even though embryos and sperm were routinely frozen before, vitrification has implied a shift in cryopreservation within assisted reproduction in Spain. Cryopreservation has gone from mainly being used for storage to being strategically used to create time windows in which to optimize IVF itself-with optimization defined by the clinics and the markets they are part of.
Vitrification allows oocyte freezing, with the accompanying expansion of "social freezing" and "egg banking," due to improved success rates of embryo freezing and means a much more flexible and adaptable technique than slow freezing. Such a combination of higher embryo success and adaptability has led to a rising use of strategic embryo freezing in Spanish clinics. This strategic freezing is expanding under cryomedicalized logics that further enable the optimization of IVF-seen as a technique that not only assists but changes the way we reproduce (Franklin 2008;Kroløkke and Bach 2020). Indeed, "IVF has been the crucible for new means of reconstructing reproduction, manipulating development, and retooling embryology" (Franklin 2013, 48), and vitrification is now crucial for facilitating this reconstruction.
The reconstruction of reproduction through vitrification is linked to reproducing later in life: people can postpone reproduction and access tools to deal with fertilization in safer ways when the time comes later in life. In this sense, vitrification can be situated within the realm of broader medicalization of reproductive aging, although it does not always aim to "rejuvenate" the body (Kroløkke and Bach 2020) but rather to optimize its reproductive capacities. The increasing technological options to deal with risk within IVF treatment, on top of the routinization of PGT, raise important questions, some of them connected to the use and abuse of selective reproduction (Gammeltoft and Wahlberg 2014) and others to the ways in which ARTs deal with and transform reproductive capacities. Both have clear effects on wider societal issues, such as those linked to neo-eugenics, or the social pressure toward fitting life cycles and intimacy into capitalist or market-driven logics.
Studying strategic freezing gives us a glimpse into the ways in which late reproduction enabled by reproductive clinics is shaping the bodies and trajectories of women in a context of reproductive crises. Reproductive aging is reformulated through freezing, including but also going far beyond, rearranging reproductive age in biographical terms (as allowed by egg freezing). Strategic freezing is also shifting how biological material coming from and going to women is understood and handled within clinics. There is a need for further social and regulatory conversations about the role of cryotechnologies in the current reproductive market in Spain. This is important because current regulation of ARTs was written before vitrification, so it does not contemplate options opened up by this now mainstream technology. These conversations could facilitate a necessary debate about what actually is taking place in the clinics: that freezing is a "helping hand" supporting a cryomedicalized mode of (gendered) remaking of fertility, risk, and age.

Author's Note
This article presents findings from the research project Suspended Life: Exploring Cryopreservation Practices in Contemporary Societies (CRYOSOCIETIES) funded by the European Research Council (Grant Agreement number: 788196; principal investigator: Thomas Lemke).

Acknowledgments
First of all, I thank all the professionals who shared their time and expertise with me. Also, I thank CRYOSOCIETIES team members-Ruzana Liburkina, Veit Braun, Thomas Lemke, and Viona Hartman-and two anonymous reviewers for their insightful critiques of earlier versions of this article. Thanks to Gerard Holden for his careful language editing and to Science, Technology, & Human Values for the review process. Finally, I need to thank Novelúa and their inhabitants for making it possible to link these words among each other.

Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: European Research Council (grant agreement number: 788196; principal investigator: Thomas Lemke).

ORCID iD
Sara Lafuente-Funes https://orcid.org/0000-0002-0299-725X Notes 1. For a fuller account of the ways in which time-lapse technology has shifted how we understand and visualize embryo development, see Chapter 4, "Aging Embryos and Viable Rhythms," in Van de Wiel (2020). 2. The reasons for Preimplantation Genetic Testing as recorded in the data could be the following: molecular diseases, cytogenetic diseases, repeated miscarriage, advanced maternal age, implantation failure, and others. 3. The Sociedad Española Fertilidad association provides the official data of cycles in Spain. It is constituted by professionals from different clinics and acts both as a lobby group and gathers data from different clinics.