The sex-specific difference in singleton birth weight after frozen embryo transfer compared with fresh embryo transfer: a secondary analysis of 3 randomized trials

Abstract

Objective: To assess whether the between-group difference in singleton birth weight following frozen vs. fresh embryo transfer varied with infant sex.

Design: A post hoc exploratory secondary analysis of data from three multicenter randomized trials compared the live birth rates between freeze-only vs. fresh embryo transfer.

Setting: Academic fertility centers.

Patient(s): A total of 1,886 women who achieved singleton live birth after a frozen or fresh embryo transfer during these trials were included.

Intervention(s): Women underwent either a frozen or fresh embryo transfer.

Main outcome measure(s): Mean birth weight, large for gestational age (LGA), and small for gestational age (SGA).

Result(s): There was an interaction between the types of embryo transfer and infant sex on the birth weight and on the incidences of LGA and SGA. Among male infants, compared with singletons following fresh embryo transfer, singletons following frozen embryo transfer had higher mean birth weights (3,520.6 ± 526.1 vs. 3,345.1 ± 524.9 g), a higher risk of being LGA (25.2% vs. 15.7%), and a lower risk of being SGA (3.3% vs. 6.1%). However, among the female infants, no statistically significant difference was found in the mean birth weight (3,336.5 ± 514.8 vs. 3,299.5 ± 485.0 g) or the risks of being LGA (18.8% vs. 15.7%) or SGA (5.2% vs. 6.0%) between frozen and fresh embryo transfer.

Conclusion(s): Male singletons born after frozen embryo transfer were more likely to have a higher birth weight than those born after fresh embryo transfer.

Keywords: Birth weight; freeze-only strategy; fresh embryo transfer; frozen embryo transfer; infant sex; peak estradiol level.