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RESEARCH QUESTION Are there greater risks of adverse perinatal outcomes, in particular of congenital malformations, after blastocyst transfer compared with cleavage-stage embryo transfer in IVF? DESIGN This was a retrospective cohort analysis from a centre for assisted reproduction at a public hospital in China over the period 2006-2015. The analysis covered all women who conceived (15,254) and newborns (16,213) from IVF/intracytoplasmic sperm injection (ICSI) cycles with cleavage-stage embryo transfer or blastocyst transfer. The principal outcome measures were congenital malformations, preterm birth (PTB), low birthweight (LBW), small for gestational age (SGA) and large for gestational age (LGA). RESULTS Logistic regression analysis showed that the sex ratio was imbalanced towards male neonates after blastocyst transfer (P=0.001; adjusted OR 1.17, 95%CI 1.07-1.30) but there were no differences in rates of miscarriage, ectopic pregnancy, stillbirth, very preterm birth (<32 weeks), PTB (<37 weeks), LBW, SGA or LGA between blastocyst transfer and cleavage-stage embryo transfer. A total of 176 congenital malformations (123 cleavage-stage embryos versus 53 blastocysts) were identified both in newborns and aborted fetuses, but the difference between groups was not statistically significant. CONCLUSIONS There was no difference in the risks of adverse perinatal outcomes, and in particular of congenital malformation, after blastocyst transfer compared with cleavage-stage transfer, although there was a sex ratio imbalance towards male neonates after blastocyst transfer.