Perinatal outcomes in singleton live births after fresh blastocyst-stage embryo 1 transfer : a retrospective analysis of 67 , 147 IVF / ICSI cycles 2 Running title : Perinatal outcomes of blastocyst-stage embryo transfer 3

21 Study question: Are perinatal outcomes different between singleton live births conceived from 22 fresh blastocyst transfer than those following the transfer of fresh cleavage-stage embryos? 23 Summary answer: Fresh blastocyst transfer does not increase risks of preterm birth, low/high 24 birth weight and congenital anomaly, alter the sex ratio at birth or prejudice the chance of a 25 having a healthy baby. 26 What is known already: Extended embryo culture is currently considered the best option for 27 embryo selection, but concerns have been raised about increased risks of preterm delivery and 28 large for gestational age (LGA) babies. 29 Study design, size, duration: We conducted a retrospective cohort study based on data from 30 the Human Fertilisation and Embryology Authority (HFEA) anonymised and cycle-based 31 dataset in the United Kingdom (UK) between 1999 and 2011. 32 Participants/materials, setting, methods: Baseline characteristics were compared between 33 blastocyst-stage and cleavage-stage embryo transfer cycles using the Chi-squared test for 34 categorical/dichotomised covariates and the Mann-Whitney test for continuous covariates. 35 Statistical significance was set at < 0.005. Poisson regression and multinomial logistic 36 regression were used to establish relationships between perinatal outcomes and blastocyst37 stage embryo transfer or cleavage-stage embryo transfer. Risk ratios (RRs), adjusted risk ratios 38 (aRRs) and their 99.5% confidence intervals (CIs) were calculated as a measure of strength of 39 associations. Results were adjusted for clinically relevant covariates. A sub-group analysis 40 included women undergoing their first IVF/ICSI treatment. Level of significance was set at < 41 0.05 and 95% CIs were calculated in the sub-group analysis. 42 Main results and the role of chance: Of a total of 67,147 IVF/ICSI cycles, 11,152 involved 43 blastocyst-stage embryo(s) and 55,995 involved cleavage-stage embryo(s). The two groups 44