Impact of maternal age on birthweight in frozen embryo transfer cycles: an analysis including 12565 singleton newborns

Background: Previous studies have investigated the effect of maternal age on assisted reproductive technology (ART) success rates. However, little is known about the relationship between maternal age and neonatal birthweight in frozen embryo transfer (FET) cycles. Does maternal age have an impact on singleton birthweight in FET cycles?Methods: This retrospective study was conducted at a tertiary care centre, involving singleton live births born to women undergoing frozen-thawed embryo transfer during the period from January 2010 to December 2017. A total of 12565 women who fulfilled the inclusion criteria were enrolled and were grouped into four groups according to the maternal age: <30, 30–34, 35-39, and ≥40 years old. Maternal age <30 years old was taken as a reference group. Singleton birthweight was the key outcome measure. A multivariable linear regression analysis was conducted to reveal the relationship between maternal age and neonatal birthweight with controlling for a number of potential confounders.Results: A modest decrease but no significant difference in birthweight and gestational age- and gender-adjusted birthweight (Z-scores) was observed in maternal age over 35 years old as compared with group with <30years old. The highest proportions of LBW (4.1%), HBW (1.2%), PTB (5.9%) and very PTB (0.9%) were found in group over 40 years old, but no significant difference was observed among four groups. Additionally, the group with 35-39 years old had the highest very LBW (0.6%) , while the group with 30-34 years old had the lowest SGA (2.7%). However, multivariate analyses revealed that neonatal outcomes including PTB, LBW and SGA were similar between the different maternal age groups.Conclusion: Grouping with different maternal age was not associated with mean birthweight and Z-scores of singletons resulting from FET.