In vitro fertilisation (IVF) and embryo transfer is an established and successful form of treatment for infertility. Recent data from the Human Fertilisation and Embryology Authority (HFEA) show consistently improving IVF success rates. The live birth rate per IVF cycle is mainly dependent on female age: during 2007, the average live birth rate per cycle started for women younger than 35 years in the UK was 32.3%, although many clinics are achieving rates of over 40%. During standard IVF treatment, in vivo matured metaphase II (MII) stage oocytes are aspirated from follicles measuring 14-20 mm in diameter. To achieve this, gonadotrophin ovarian stimulation is used, most commonly after achieving pituitary suppression using a gonadotrophin-releasing hormone (GnRH) agonist (long-protocol IVF).

However, these high rates of success are achieved at the expense of two major complications, namely multiple pregnancy and ovarian hyperstimulation syndrome (OHSS). Over 25% of the IVF live births described above were multiple. This results from the fairly routine transfer of multiple embryos to the uterine cavity in an attempt to overcome low implantation rates, particularly in older women. Moreover, in order to have multiple embryos available for transfer, ovarian stimulation is required, which places the woman at risk of developing OHSS. There has been much recent focus on reducing both the rate of multiple pregnancy and also the rate of OHSS. Prolonged embryo culture to day 5 or 6 (blastocyst stage) with replacement of a single embryo maintains pregnancy rates while significantly reducing the multiple pregnancy rate. However, the rate of embryo attrition means that multiple oocytes need to be retrieved and successfully fertilised to allow sufficient good-quality day 3 embryos to continue culture to day 5.