Effect of endometrial thickness on birthweight in frozen embryo transfer cycles: an analysis including 6181 singleton newborns.

STUDY QUESTION

Does endometrial thickness (EMT) have an impact on singleton birthweight in frozen embryo transfer (FET) cycles?

SUMMARY ANSWER

An EMT <8 mm was associated with a lower mean birthweight and gestational age- and gender-adjusted birthweight (Z-scores) of singletons resulting from FET.

WHAT IS KNOWN ALREADY

Previous studies have examined the impact of EMT on IVF success rates. Little is known, however, regarding the relationship between EMT and neonatal birthweight.

STUDY DESIGN, SIZE, DURATION: This retrospective study involved singleton live births born to women undergoing frozen-thawed Day 3 embryo transfer during the period from January 2010 to December 2017 at a tertiary care centre.

PARTICIPANTS/MATERIALS, SETTING, METHODS: A total of 6181 women who fulfilled the inclusion criteria were included and were grouped into five groups depending on the EMT: <8 mm, 8-9.9 mm, 10-11.9 mm, 12-13.9 mm and ≥14 mm. EMT between 10 and 11.9 mm was taken as a reference group. Singleton birthweight was the primary outcome measure. A multivariable linear regression analysis was performed to detect a relationship between EMT and newborns' birthweight after controlling for a number of potential confounders.

MAIN RESULTS AND THE ROLE OF CHANCE

A modest but significant decrease in birthweight was observed in the EMT <8 mm group as compared with groups with EMT ≥10 mm, with a mean difference of 89-108 g. Also, singletons from the EMT <8 mm group (0.24 ± 1.04) had a significantly lower birthweight Z-scores than those from the EMT 10-11.9 mm (0.41 ± 1.02; P = 0.032) or EMT 12-13.9 mm (0.46 ± 1.07; P = 0.004) groups. Further, multiple linear regression analyses indicated that parental BMIs, gestational age, newborn gender, pregnancy complications and EMT <8 mm were all independent predictors of neonatal birthweight.

LIMITATIONS, REASONS FOR CAUTION: The present study was limited by its retrospective design. Future prospective studies are required to confirm our findings.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings provided new insight into the relationship between EMT and neonatal outcomes by showing that a thin endometrium is associated with a decrease in singleton birthweight.

STUDY FUNDING/COMPETING INTEREST(S): National Key Research and Development Program of China (2018YFC1003000); the National Natural Science Foundation of China (81771533, 81571397, 31770989, 81671520); the China Postdoctoral Science Foundation (2018M630456). The authors have no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

Not applicable.