Relationships between birth weight discordance and maternal and perinatal risks among twin pregnancies conceived following frozen embryo transfer

STUDY QUESTION Is intertwin birth weight discordance associated with adverse maternal and perinatal outcomes following frozen embryo transfer (FET)? SUMMARY ANSWER For twins conceived following FET, intertwin birth weight discordance is related to elevated risks of neonatal mortality irrespective of chorionicity, and the risk of hypertensive disorders of pregnancy (HDP) is elevated for the mothers of dichorionic twins affected by such birth weight discordance. WHAT IS KNOWN ALREADY While the relationships between intertwin birth weight discordance and adverse maternal or fetal outcomes have been studied for naturally conceived twins, similarly comprehensive analyses for twins conceived using ART remain to be performed. STUDY DESIGN, SIZE, DURATION This was a retrospective cohort study of all twin births from 2007 to 2021 at Shanghai Ninth People's Hospital in Shanghai, China that were conceived following FET (N = 6265). PARTICIPANTS/MATERIALS, SETTING, METHODS Intertwin birth weight discordance was defined as a 20% difference in neonatal birth weights. The primary study outcome was the incidence of HDP and neonatal death while secondary outcomes included gestational diabetes, placenta previa, placental abruption, intrahepatic cholestasis of pregnancy, preterm premature rupture of the membranes, Cesarean delivery, gestational age, birth weight, stillbirth, birth defect, neonatal jaundice, necrotizing enterocolitis, and pneumonia incidence. Logistic regression models were used to estimate adjusted odds ratios (aORs) and 95% CIs for maternal and neonatal outcomes. Subgroup analyses were conducted, and Kaplan-Meier survival analysis was used to estimate the survival probability. The sensitivity analysis was performed with a propensity score-based patient-matching model, an inverse probability weighting model, a restricted cubic spline analysis, and logistic regression models using other percentage cutoffs for discordance. MAIN RESULTS AND THE ROLE OF CHANCE Of 6101 females that gave birth to dichorionic twins during the study interval, birth weight discordance was observed in 797 twin pairs (13.1%). In this cohort, intertwin birth weight discordance was related to an elevated risk of HDP (aOR 1.56; 95% CI 1.21-2.00), and this relationship was confirmed through sensitivity analyses. Hypertensive disease risk rose as the severity of this birth weight discordance increased. Discordant birth weight was also linked to increased odds of neonatal mortality (aOR 2.13; 95% CI 1.03-4.09) and this risk also increased with the severity of discordance. Of the 164 women with monochorionic twins, the discordant group exhibited an elevated risk of neonatal death compared to the concordant group (crude OR 9.00; 95% CI 1.02-79.3). LIMITATIONS, REASONS FOR CAUTION The limitations of this study are its retrospective nature and the fact that the available data could not specify which twins were affected by adverse outcomes. There is a lack of an established reference birth weight for Chinese twins born at a gestational age of 24-41 weeks. WIDER IMPLICATIONS OF THE FINDINGS These findings suggest that twins exhibiting a birth weight discordance are related to an elevated risk of adverse maternal and perinatal outcomes, emphasizing a potential need for higher levels of antenatal surveillance in these at-risk pregnancies. STUDY FUNDING/COMPETING INTEREST(S) Authors declare no conflict of interest. This study was funded by the Clinical Research Program of Shanghai Ninth People's Hospital affiliated to Shanghai Jiao Tong University School of Medicine (JYLJ202118) and the National Natural Science Foundation of China (Grant Nos 82271693 and 82273634).