Delayed blastocyst development does not influence the outcome of frozen–thawed transfer cycles T El-Toukhy, E Wharf, R Walavalkar, A Singh, V Bolton, Y Khalaf, P Braude Assisted Conception Unit, Guy’s and St. Thomas’ Hospital NHS Foundation Trust, London, UK Correspondence: Dr T El-Toukhy, Assisted Conception Unit, 11th Floor Tower Wing, Guy’s Hospital, St. Thomas Street, London SE1 9RT, UK. Emails tarek.el-toukhy@gstt.nhs.uk; tarekeltoukhy@hotmail.com Accepted 12 July 2011. Published Online 6 September 2011. Objective To compare the outcome of transfer of thawed blastocysts frozen on either day 5 or day 6 after in vitro fertilisation. Design Cohort observational study. Setting Tertiary assisted conception unit in London, UK. Population Six hundred and forty-two consecutive nondonor programmed thawed blastocyst transfer (TBT) cycles. Methods High-grade blastocysts were frozen on day 5 (n = 314) or day 6 (n = 328) after fertilisation using a slow-freezing protocol. Endometrial preparation was performed using estradiol valerate. Progesterone supplementation was commenced when the endometrial thickness had reached 7 mm or more. Frozen blastocysts were thawed on day 6 of progesterone supplementation and assessed immediately after thawing for survival, and after 3–4 hours for blastocoele re-expansion. Main outcome measures Thawed blastocyst survival and re-expansion rates, and pregnancy and live birth rates, per TBT. Results Thawed blastocyst survival and re-expansion rates were comparable between the day 5 and day 6 groups (87% versus 87%, P = 0.50 and 73% versus 71%, P = 0.35, respectively). The live birth rate was similar between the two groups (29% versus 28.5%, P = 0.93, respectively). After adjusting for confounding variables, the odds ratio (OR) of a live birth in cycles in which the thawed blastocysts were frozen on day 6 compared with day 5 was 1.23 [95% confidence interval (CI), 0.81–1.86, P = 0.34]. Conclusion The pregnancy potential of high-grade blastocysts frozen on day 5 and day 6 after in vitro fertilisation and replaced in programmed TBT cycles is comparable. Keywords Blastocyst, blastocyst transfer, embryo development, embryo freezing, endometrium, in vitro fertilisation outcome. Please cite this paper as: El-Toukhy T, Wharf E, Walavalkar R, Singh A, Bolton V, Khalaf Y, Braude P. Delayed blastocyst development does not influence the outcome of frozen–thawed transfer cycles. BJOG 2011;118:1551–1556. Introduction Extended embryo culture to the blastocyst stage has per- mitted a better appreciation of embryo quality and improved pregnancy rates after in vitro fertilisation (IVF) treatment. It has also allowed the transfer of fewer embryos to reduce the risk of multiple pregnancy, leav- ing more surplus embryos available for cryopreservation. Consequently, optimum utilisation of all available blast- ocysts is imperative and establishment of an efficient blastocyst freezing programme is an integral part of such a strategy. 1 Approximately one-half of IVF embryos will reach the blastocyst stage on day 5 or day 6 after fertilisation. 2 In the fresh IVF cycle, the pregnancy rate after transfer of day 5 blastocysts is superior to that achieved after transfer of the slower developing day 6 blastocysts. 3 However, the impact of this developmental delay on the pregnancy potential of frozen blastocysts is unclear. Although several researchers have suggested that the transfer of thawed day 5 blastocysts is associated with a higher pregnancy rate compared with the transfer of thawed day 6 blastocysts, 4–6 others have reported comparable outcomes 7–9 and a few have observed a higher pregnancy rate after the transfer of thawed day 6 blastocysts. 10,11 A recent critical review has revealed that the majority of these studies had a retrospective design, lacked sufficient power to detect meaningful differences and did not con- sider important confounders, such as female age, outcome of the fresh treatment cycle, blastocyst grade at cryopreser- vation and number of blastocysts transferred. 12 The review recommended the performance of well-designed large ª 2011 The Authors BJOG An International Journal of Obstetrics and Gynaecology ª 2011 RCOG 1551 DOI: 10.1111/j.1471-0528.2011.03101.x www.bjog.org Fertility and assisted reproduction