Effect of embryo and blastocyst transfer on the birthweight of live-born singletons from FET cycles

Purpose To evaluate the effect of culture duration (embryo (day 3) transfer vs. blastocyst (day 5–6) transfer) on the birthweight of singletons from frozen embryo transfer (FET) cycles. Methods A total of 1092 singletons were analyzed in this retrospective study. The distribution of large for gestational age (LGA) infants, the mean birthweight, and z scores of singletons were compared between the day 3 and day 5–6 transfer groups. Multiple linear regression analysis was performed to evaluate the relationships between confounding factors and singleton birthweight. Results The proportion of LGA infants significantly increased with BMI (BMI < 20, 12.8%; 20 ≤ BMI ≤ 25, 23.2%; BMI > 25, 32.3%; P < 0.0001). However, the proportions of small for gestational age (SGA) and LGA infants were not significantly different between day 3 and day 5–6 transfers. The absolute mean birthweight of singletons was not significantly different between day 3 transfer (3422 ± 547 g) and day 5–6 transfer (3433 ± 559 g; P = 0.732). The z scores (calculated from a reference population) of singletons were also not significantly different between the two groups (0.499 vs. 0.533, P = 0.625). Multiple linear regression analysis showed that maternal BMI, gestational age, and infant gender had significant effects on singleton birthweight, while culture duration ( P = 0.731) did not significantly affect singleton birthweight. Conclusions In vitro culture duration did not affect the birthweight of newborns resulting from day 3 to day 5–6 transfers in FET cycles.