Persisting risk factors for preeclampsia among high-risk pregnancies already using prophylactic aspirin: a multi-country retrospective investigation

Abstract Background Low-dose aspirin is recommended for pregnant individuals at high-risk of developing preeclampsia, but less is known about those that develop preeclampsia even while using prophylactic aspirin for preeclampsia prevention as the best course of treatment. Objectives The objective of this study is to investigate the risk factors with the highest risk of developing preeclampsia among pregnant individuals already using aspirin from high-risk obstetrical centers across five countries. Design This is a secondary analysis of pregnant individuals from the Folic Acid Clinical Trial (FACT) who were using prophylactic aspirin before 16 weeks gestation. The FACT randomized control trial took place in 70 high risk obstetrical centers in Canada, United Kingdom, Australia, Jamaica, and Argentina between 2011–2015. Participants were included if they had any of the risk factors for preeclampsia: diabetes, chronic hypertension, twin pregnancy, history of preeclampsia, and/or obesity (Body Mass Index ≥35). The outcomes of interest were preeclampsia and preterm preeclampsia (<37 weeks). Log binomial regressions assessed factors significantly associated with any preeclampsia or preterm-preeclampsia (<37 weeks) using adjusted risk ratios (ARR) and 95% confidence intervals (CI). Results There were 2296 pregnant individuals with complete information on aspirin included in this study. At baseline, all patients were at high risk of preeclampsia and were eligible for aspirin prophylaxis, however, only 660 (28.7%) were taking aspirin. Among the 660 pregnant individuals taking aspirin, 132 (20%) developed preeclampsia and 60 (9.09%) preterm preeclampsia. Among pregnant individuals using aspirin, the risks of preeclampsia were highest for twins (ARR:2.62, 95% CI: 1.68–4.11), history of preeclampsia (ARR: 2.42, 95% CI: 1.74–3.38), and hypertension (ARR:1.92, 95% CI: 1.37–2.69). Similar trends were found for preterm-preeclampsia for twins (ARR:4.10, 95% CI:2.15–7.82), history of preeclampsia (ARR:2.75, 95% CI:1.62–4.67), and hypertension (ARR:2.18, 95% CI:1.28–3.72). No significant differences were found for obesity or diabetes. Conclusion These findings suggest that individuals with twin pregnancies, a history of preeclampsia, or hypertension may not benefit from aspirin to the same extent as those with other complications such as obesity or diabetes. Careful clinical monitoring for these risks factors is recommended and future research into the effectiveness in these populations would increase our understanding of the current best practice of prophylactic aspirin use to prevent preeclampsia. TRIAL REGISTRATION: Current Controlled Trials ISRCTN23781770 and ClinicalTrials.gov NCT01355159


Introduction
Globally, preeclampsia affects 2.1-5.6% of pregnancies [1]. Preeclampsia is a progressive, multisystem disorder of pregnancy characterized by new-onset hypertension and proteinuria or hypertension with significant end-organ dysfunction [2]. Preeclampsia is a significant source of maternal and fetal morbidity and is the second most common cause of maternal mortality globally [2][3][4][5]. Short-term consequences of preeclampsia include increased risk of maternal antenatal stroke, renal or hepatic impairment, pulmonary edema, hematologic dysfunction (including hemolysis and thrombocytopenia), neurologic involvement, intrauterine growth restriction (IUGR), and preterm delivery [6][7][8]. Long-term consequences include increased risk of cardiovascular and cerebrovascular disease for both the pregnant person and infant, as well as reduced cognitive function and development of sequelae from IUGR and preterm delivery for infants [6].
While the only definitive treatment for preeclampsia is delivery, clinical management requires a balance between minimizing risk to the pregnant person and the fetal risk of prematurity [3]. Emphasis is placed on early identification of pregnant individuals at increased risk of disease development and administration of prophylactic interventions and managing preexisting conditions. A Cochrane systematic review was published first in 2003 [9] and then updated in 2007 [10] and 2019 [11] (now currently synthesizing 77 trials) consistently reporting a 17-19% reduction in the risk of preeclampsia associated with the use of antiplatelet agents, concluding that prophylactic use of low-dose aspirin led to small-to-moderate benefits.
Informed by the current evidence, the World Health Organization [12], the US Prevention Services Task Force (USPSTF) [13], and national guidelines for the management of hypertension in pregnant individuals in Canada [14], the United Kingdom [15], Australia [16], and the United States [17] recommend prophylactic use of low-dose aspirin in pregnant individuals at high risk of developing preeclampsia [4,13,14,18]. From a practical standpoint, low-dose aspirin is effective, easily administered, affordable, and widely available in multiple settings, including high, middle, and low-income countries [19]. High-risk factors for preeclampsia that warrant low-dose aspirin treatment include a previous past history of preeclampsia, multifetal gestation, chronic hypertension, diabetes mellitus, or certain autoimmune diseases [13]. Additional moderate-risk factors include maternal obesity, maternal age 35 years, or family history of preeclampsia [13].
Many high-risk pregnant individuals take prophylactic aspirin but still go on to develop preeclampsia. The objective of this study is to identify the risk factors for preeclampsia that persist even while using prophylactic aspirin.

Study context, design and sample
This study is a secondary analysis of data from the Folic Acid Clinical Trial (FACT). FACT was an multi-country, multicenter, randomized control trial in 70 high risk obstetrical centers in Canada, United Kingdom, Australia, Jamaica and Argentina between 2011 and 2015 [21]. FACT investigated the effect of folic acid on preeclampsia by comparing a placebo group (up to 1.1 mg dosage of folic acid) to high-dose folic acid (4.0 to 5.1 mg/day). In brief, the trial was null and found that high dose folic acid compared to low dose did not significantly reduce the incidence of preeclampsia among high-risk pregnant individuals.
This current study leverages the large sample of pregnant individuals at high risk of developing preeclampsia enrolled in FACT to observationally investigate patterns of aspirin use. This sample is restricted to pregnant individuals with complete data on aspirin use taking any daily dose of aspirin before 16 weeks of gestation through to delivery. A summary of global guidelines for aspirin use is available in Appendix 1.

Outcome measures
The primary outcomes of interest were preeclampsia and preterm preeclampsia (before 37 weeks of completed gestation). The trial protocol definition of preeclampsia was: diastolic blood pressure 90 mm Hg on two occasions four hours or more apart and proteinuria (more than þþ on dipstick, or urinary protein 300 mg in 24 h urine collection, or random protein: creatinine ratio 30 mg protein/mmol) in pregnant individuals at 20 weeks of gestation or greater, or diagnosis of HELLP syndrome (hemolysis, elevated liver enzymes, low platelets) or superimposed preeclampsia (history of preexisting hypertension diagnosed before pregnancy or before 20 weeks' gestation with new proteinuria [20]. The final diagnosis was adjudicated by a clinical review board.

Demographic and clinical variables
Five risk factors for preeclampsia were investigated including preexisting chronic hypertension, diabetes (Type 1 or Type 2), history of preeclampsia, obesity defined as a Body Mass Index (BMI) over 35 kg/m 2 , and twin pregnancy. Documentation of BMI three months before pregnancy and up to the time of randomization was required as part of study eligibility.
Demographic factors included: country site, maternal ethnicity, marital status, education level, and age. Age was measured continuously, categorically (<20, 20-29, 30-34, 35 years), and dichotomized to capture advanced maternal age (35 years). Obstetrical factors included parity (nulliparous vs. parous), smoking, or alcohol use during pregnancy (yes, no, or quit during pregnancy). Prescriptions and supplements include folic acid comparing low dose (1.1 mg/day) vs high dose (4 mg/day), any calcium supplements, and aspirin use comparing low dose (75-81mg per day) vs high dose (82-150 mg per day). Only folic acid was administered as part of the trial, both calcium and aspirin were self-reported.

Statistical analyses
All statistical analyses were conducted using SAS software (version 9.4). Descriptive statistics included totals and percentages for categorical variables. Continuous variables were displayed with medians (med) and interquartile ranges (IQR).
Bivariable log binomial regressions were built to investigate the five main risk factors (i.e. chronic hypertension, diabetes, history of preeclampsia, BMI 35, and twin pregnancy) and identify the factors with the highest risks of developing preeclampsia while taking prophylactic aspirin. The multivariable models included the five risk factors while adjusting for age, aspirin dose, and country site. In the event that the model did not converge, the variables were reviewed, and the final model included factors to minimize the Akaike Information Criteria (AIC) of the model. Estimates include risk ratios (RR), adjusted risk ratios (ARR), and 95% confidence intervals (CI).
FACT was approved by the Ottawa Health Sciences Research Ethics Board (2009107), and at all participating trial sites. FACT was funded by the Canadian Institutes of Health Research (grants 198801 and 98030).
In Table 3 In Table 4, the highest incidence of preterm preeclampsia was documented among 35 (58.33%) with a history of preeclampsia, 22 (36.67%) with chronic hypertension, and 18 (30.00%) with obesity. At the multivariable level, the full model (five risk factors, age, and country) did not converge, and the model was selected based on minimizing the AIC (and only included chronic hypertension, history of preeclampsia, and twin pregnancy). The highest risk for preeclampsia was seen among those with twin pregnancies (ARR:4.10, 95% CI:2.15-7.82), a history of preeclampsia (ARR:2.75, 95% CI:1.62-4.67), and chronic hypertension (ARR:2.18, 95% CI:1.28-3.72).

Discussion
This study has found that among a sample over 600 pregnant individuals using prophylactic aspirin for preeclampsia, 20% went on to develop preeclampsia and 9% preterm preeclampsia. Even while taking aspirin, those with twin pregnancies, history of preeclampsia, or chronic hypertension emerged as groups with the highest risk of developing preeclampsia. There were no significant differences in risk for those with obesity, diabetes, country, or age. The incidence of both term and preterm preeclampsia found in this study is comparable to those identified in other studies with similar at-risk groups. In a large US trial conducted by the National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units, the incidence of preeclampsia was 18% in high-risk individuals receiving aspirin therapy [21]. Recently, a large multicenter, placebo-controlled trial identified that preterm preeclampsia occurred in 1.6% of study participants receiving daily low-dose aspirin prophylaxis [22]. The study assessed singleton pregnancies using an algorithm that combined maternal factors with mean arterial pressure, uterine-artery pulsatility index, and maternal serum pregnancy-associated plasma protein A and placental growth factor to identify pregnant individuals at high risk for preeclampsia. Given the inclusion of multiparous gestation and adherence to clinical risk stratification, it is likely that the incidence of preterm preeclampsia among high-risk pregnant individuals receiving aspirin therapy identified in this current study maybe a more generalizable estimate given the broader inclusion criteria [23].
The findings from this study call attention to the sustained risk for preeclampsia and identify clinical risk factors whereby increased clinical monitoring or supplemental therapy may be indicated. Of interest, many published trials exclude twin pregnancies and our study identified that twin pregnancies had 2 to 4 times the risk of developing preeclampsia [22,24,25]. The inclusion of twins and the analyses of twin data can be methodologically complex, however, this study highlights the increased risk and supports the importance of including multi-fetal gestation in the clinical evaluation of preeclampsia [26].
A meta-analysis of large cohort studies investigating the association and relative risk of preeclampsia identified prior preeclampsia and chronic hypertension as having the strongest effect [27], followed by pre-gestational diabetes and multifetal pregnancy. Interestingly, preexisting diabetes was not identified in the present study as a significant risk factor for  preeclampsia. This may be due to the complex interplay between aspirin and glycemic control [28,29], or that there were fewer people with diabetes in the trial, or potentially attributed to limited information on adherence to treatment for diabetes and diet within the FACT Trial. [30] Out of all FACT participants, fewer than one-third were taking a daily dose of aspirin before 16 weeks of gestation [20]. This raises the question of barriers to aspirin prescribing patterns, uptake, and adherence. FACT enrolled participants between 2011-2015, before several RCTs were published and before clinical guidelines recommended low-dose aspirin as an effective intervention for preeclampsia prophylaxis. Inconsistent prescribing patterns may be due in part to the division regarding its efficacy as some studies at the time had reported no significant effect on the risk of preeclampsia [31]. Recently published research has provided conflicting evidence regarding the efficacy of aspirin for hypertension and the prevention of preeclampsia [32][33][34]. Non-adherence with medical therapy is a multifaceted and complex problem involving patient factors such as physician preferences, regimen factors, personal, family, cultural, demographic factors, geographic and socioeconomic barriers to accessing treatment, [35]. In addition to the multitude of factors that influence medication adherence in non-pregnant individuals, the unique factor of concern for potential fetal harm has been identified as a barrier to medication adherence within the pregnant population [35][36][37][38].  .788 a Multivariable model did not converge, the variables were reviewed, and the final model included factors to minimize the Akaike Information Criteria (AIC) of the mode.

Limitations
A notable strength of this work is the large, international, multicenter design of the FACT trial, permitting the investigation of preeclampsia risk factors across five countries. There was high data completeness and the follow-up rate for all participants was greater than 95% [20]. Limitations include the potential for measurement bias as aspirin use was selfreported at follow-up visits. As this is a secondary analysis from FACT (focused on adherence to folic acid), there was limited information on the timing and dose of aspirin. Confounding by indication is possible as pregnant individuals at high risk for preeclampsia were the most likely to be placed on aspirin. Further research can investigate as to whether alterations to dose and timing of aspirin might mitigate risk.

Conclusions
While aspirin continues to be the best-recommended course of treatment in the prevention of preeclampsia among pregnant individuals at high risk, 20% and 9% still went on to develop preeclampsia and pretermpreeclampsia, even while taking aspirin. While many robust studies have proven the protective effect of aspirin, the literature is becoming divided and this study highlights that a large percentage of women still develop preeclampsia, and a continued search for preventative treatments and symptom management are still needed in addition to the current recommendations of low-dose aspirin.