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.
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.