High birth weight and greater gestational age at birth in singletons born after frozen compared to fresh embryo...

While many studies agree that the fetal birth weight is higher after frozen embryo transfer (FET), few studies have explored the difference in fetal weight change during such pregnancies. This cohort study was to identify the difference in fetal birth weight and gestational age at birth between singletons born following fresh ET and those born following FET.

This was a hospital-based cohort study using clinical data from the Kaohsiung Chang Gung Memorial Hospital Obstetric and Neonatal Database 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.

Compared with those in the fresh ET group, singletons in the FET group had higher birth weight (3137 g [2880-3441 g] vs. 3060 g [2710-3340 g], p < 0.05), were born later (39.0 weeks of gestation [38.0-40.0 weeks] vs. 38.0 weeks of gestation [37.0-39.0 weeks], p < 0.05), and had a lower incidence of preterm birth (10.4% vs. 15.2%, p < 0.05). The difference in birth weight was not associated with maternal body weight (BW) or body mass index, increase in maternal BW in the third trimester, but related to the total increase in maternal BW during pregnancy.

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.