Risk of preterm birth after blastocyst embryo transfer: a large population study using contemporary registry data from Australia and New Zealand

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To investigate whether there is an increased risk of preterm birth with blastocyst transfer compared with cleavage-stage embryo transfer (ET) after assisted reproductive technology (ART). A retrospective, population-based study. Not applicable. A total of 50,788 infants conceived after ART treatment performed from 2009 to 2012. None. The rates of preterm birth, low birth weight (LBW), and small for gestational age (SGA) for 43,952 singleton and 3,418 twin deliveries after transfers of blastocyst or cleavage-stage embryos. Among singletons, there was no significant difference in the odds of preterm birth between blastocyst and cleavage-stage ET (9.1% compared with 9.3%, respectively, adjusted odds ratio 1.00, 95% confidence interval 0.94-1.08). Among twins, the crude rates of preterm birth were similar after blastocyst and cleavage-stage ETs (61.5% and 64.4%, respectively). However, after adjusting for potential confounders, blastocyst transfer was associated with a lower odds of preterm birth among twins (adjusted odds ratio 0.80, 95% confidence interval 0.70-0.93). There was no difference in risks of LBW and SGA between blastocyst and cleavage-stage ETs for both singletons and twins after adjusting for potential confounders. In contrast with the findings from a number of other studies, blastocyst culture in Australian and New Zealand is not associated with an increased risk of preterm, LBW, and SGA for singletons. Further studies are needed to assess longer-term outcomes of children born after ART treatment and possible biological or treatment factors related to adverse outcomes.