Secondary sex ratio in assisted reproduction: an analysis of 1 376 454 treatment cycles performed in the UK.

study questionDoes ART impact the secondary sex ratio (SSR) when compared to natural conception? SUMMARY ANSWER: IVF and ICSI as well as the stage of embryo transfer does impact the overall SSR. WHAT IS KNOWN ALREADY: The World Health Organization quotes SSR for natural conception to range between 103 and 110 males per 100 female births. STUDY DESIGN SIZE DURATION: A total of 1 376 454 ART cycles were identified, of which 1 002 698 (72.8%) cycles involved IVF or ICSI. Of these, 863 859 (85.2%) were fresh cycles and 124 654 (12.4%) were frozen cycles. Missing data were identified in 14 185 (1.4%) cycles. PARTICIPANTS/MATERIALS SETTING

methodsAll cycles recorded in the anonymized UK Human Fertilisation and Embryology Authority (HFEA) registry database between 1991 and 2016 were analysed. All singleton live births were included, and multiple births were excluded to avoid duplication. MAIN RESULTS AND THE ROLE OF CHANCE: The overall live birth rate per cycle for all IVF and ICSI treatments was 26.2% ( LIMITATIONS REASONS FOR CAUTION: The quality of the data relies on the reporting system. Furthermore, success rates through ART have improved since 1991, with an increased number of blastocyst stage embryo transfers. WIDER IMPLICATIONS OF THE

findingsThis is the largest study to date evaluating the impact of ART on SSR. The results demonstrate that, overall, ART does have an impact on the SSR when assessed according to the method of fertilization (ICSI increased female births while IVF increased males). However, given the ratio of IVF to ICSI cycles at present with 60% of cycles from IVF and 40% from ICSI, the overall SSR for ART closely reflects the population SSR for, largely, natural conceptions in England and Wales. STUDY FUNDING/COMPETING INTERESTS: The study received no funding. C.M.B. is a member of the independent data monitoring group for a clinical endometriosis trial by ObsEva. He is on the scientific advisory board for Myovant and medical advisory board for Flo Health. He has received research grants from Bayer AG, MDNA Life Sciences, Volition Rx and Roche Diagnostics as well as from Wellbeing of Women, Medical Research Council UK, the NIH, the UK National Institute for Health Research and the European Union. He is the current Chair of the Endometriosis Guideline Development Group for ESHRE and was a co-opted member of the Endometriosis Guideline Group by the UK National Institute for Health and Care Excellence (NICE). I.G. has received research grants from Wellbeing of Women, the European Union and Finox. TRIAL REGISTRATION NUMBER: Not applicable.