Supplementary Material for: Fresh and Frozen-Thawed Embryo Transfer Compared to Natural Conception: Differences in Perinatal Outcome?

Background/Aim: To compare the pregnancy outcome of singletons conceived after transfer of cryopreserved and thawed embryos (frozen embryo transfer [FET]) to singletons conceived after fresh embryo transfer (fresh ET) and natural conceived singletons. Methods: Using a retrospective data analysis on a study population consisting of 1,261 singletons born after FET and 2,519 singletons born after fresh ET between 2006 and 2015. The control group consisted of singletons born after natural conception. Main outcome measures consisted of birth weight (in grams), gestational age, preterm birth (<37 weeks of gestation), being large for gestational age (LGA, above 90th weight percentile adjusted for gestational age) and Apgar scores. Results: Babies born after FET had an increased risk of high birth weight (adjusted OR [AOR]) 2.92; 1.503-3.482) and being LGA (AOR fresh ET vs. FET 1.47; 1.210-1.787) compared to singletons born after fresh ET, as well as higher birth weights compared to natural conceived children. Conclusions: Singletons born after FET have a higher risk of high birth weight and being LGA compared to singletons after fresh ET and compared to natural conceived singletons. We assume that the freezing process might be the underlying cause.