Increased Risk of High Birthweight in Singleton Newborns after Frozen-Thawed Embryo Transfer According to Endometrial Preparation, Ovulatory or Artificial Cycle

Background: It is unknown whether prolonged artificial hormonal environment during early fetal development affects the birthweight of singletons born after frozen-thawed embryo transfer (FET). Methods: A retrospective observational study included singleton births>22 weeks of gestation obtained after FET between 2013-2019 after endometrial preparation with ovulatory cycle (OC) or artificial cycle (AC). Our primary objective was to compare birthweight of singletons after FET between endometrial preparation by OC or AC. Secondary objectives included prolonged pregnancies, high birthweight, low birthweight, SGA and LGA rates. Multivariate analyses were performed considering potential confounding factors. Results: Among 198 singleton live births after FET, 112 were obtained with OC and 86 with AC. Prolonged pregnancies rate was higher in AC (25.6% vs. 7.1%, respectively, p=0.001). Mean birthweight was higher (+219g) in AC (3386g vs. 3167g, p=0.01; adjusted-p=0.052), as well as the rate of babies exceeding 4000g (16.3% vs. 2.7%, p=0.03, adjusted-p=0.015). The rate of babies <2500g was lower in AC (3.5% vs. 11.6%, respectively, p=0.050, adjusted-p=0.049). Conclusions: Since OC does not strain the chances of pregnancy and in the incomplete knowledge of the consequences of neonatal overweight on the future health of children, OC preparation could be advocated as first-line endometrial preparation in FET.