The Difference in Birthweight in Singletons Born After Fresh Embryo Transfer and After Frozen Embryo Transfer Becomes Significant in the Late Third Trimester

Background: Although it seems to be the consensus that the birth weight of the fetus has increased in FET, few studies have explored the pattern of fetal weight change during pregnancy. The purpose of this study is to explore when fetal weight begins to differ between singletons born after fresh embryo transfer (ET) and those born after frozen embryo transfer (FET).Methods: This was a hospital-based cohort study using clinical data from the Kaohsiung Chang Gung Memorial Hospital Obstetric and Neonatal Database (KCGMHOND) from January 1, 2007, to December 1, 2018. A sample of 784 eligible women who had singleton pregnancies and live-born deliveries after 428 fresh ET or 356 FET between January 2007 and December 2018. Results: Compared to those in the fresh ET group, singletons in the FET group had higher birthweight (3137 g [2880-3441] vs. 3060 g [2710-3340], P<0.05), were born later (39.0 weeks of gestation [38.0-40.0] vs. 38.0 weeks of gestation [37.0-39.0], P<0.05) and had a lower ratio of preterm birth (10.4% vs. 15.2%, P<0.05).The difference in birthweight was not associated with maternal body weight (BW) and body mass index (BMI), maternal BW gained in the third trimester or total BW gained during pregnancy. When second- and third-trimester gestational ages were compared by estimated date of delivery (EDD, days) using ultrasound measurement, there were also no significant differences.Conclusions: The birthweight of singletons born following FET and fresh ET became significant in the late third trimester. The main reason is that singletons conceived from FET were at a lower relative risk of preterm delivery and had a higher gestational age at birth.