Objective To test the hypothesis that the fetal fraction in twin pregnancy would reflect birthweight discrepancy. Design Retrospective cohort study. Setting International Peace Maternity and Child Health Hospital in Shanghai, China Population We included 237 twin pregnancies undergoing cfDNA screening for aneuploidy and delivered at International Peace Maternity and Child Health Hospital in Shanghai, China between January 2018 to December 2021. Exclusion criteria including abnormal NIPT results， fetal demise or structural abnormalities. Methods All women with twin pregnancies were offered a scan to determine chorionicity as well as first-trimester nuchal translucency (NT) at 11 +0-13 +6 weeks. Dichorionic was confirmed when ultrasound assessment clearly indicates two placentas. The twin peak sign is used to distinguish chorionicity if only one placenta is visualized. Main Outcome Measures Fetal fraction and birth weight of the new borns were collected. Relationships between fetal fraction and birth weight discordance or sFGR were analysed. Results Fetal fraction was positively correlated with the difference of birth weight (β=0.004, 95%CI: 0.001~0.006). Higher fetal fraction was significantly associated with the increased risk of birthweight discordance of 20% (adjusted OR:1.073, 95%CI: 1.009~1.142) and 25% (adjusted OR:1.092, 95%CI: 1.006~1.185) and sIUGR(adjusted OR:1.130, 95%CI: 1.038~1.231). We obtained the optimum cut-off point of fetal fraction ≥ 11.790, ≥ 14.800 and ≥ 14.800 for birthweight discordance of 20% and 25% and sFGR, respectively. Conclusion This study shown that fetal fraction was positively correlated with the difference of birth weight. Fetal fraction could be used as a biomarker in predating birth weight discordance and sFGR, and help to make individualized clinical monitoring plans for twin pregnancies.

Objectives: We attempted to establish a set of fetal biometric references for Chinese twin pregnancies, stratified by chorionicity and conception mode as spontaneously conceived monochorionic diamniotic (SC-MCDA), spontaneously conceived dichorionic diamniotic (SC-DCDA) and assisted reproductive technology dichorionic diamniotic (ART-DCDA). Design: Retrospective cohort study. Setting: International Peace Maternity & Child Health Hospital, shanghai, China. Population or Sample: 929 twin pregnant women, along with 2019 singleton pregnant women, were qualified for the inclusion criteria. Methods: The linear mixed models were used to test the difference of growth pattern between groups, and the growth curve of each biometric parameter was modeled by generalized additive model for location scale and shape. Main Outcome Measures: From 2016 to 2019, ultrasonographic fetal biometric measurements were longitudinally collected in pregnant women, in which six measurements were recorded: fetal weight, biparietal diameter, head circumference, abdominal circumference, femur length and humerus length. Results: Overally, SC-DCDA twins grew faster than SC-MCDA twins, while slower than ART-DCDA twins. Correspondingly, the week-specific differences between groups were also identified for all the six biometric measurements, though the differences were not observed in all gestational weeks. The customized fetal growth charts of each fetal biometric parameter were then respectively constructed for SC-MCDA, SC-DCDA and ART-DCDA twins, and all of the three shows significant difference from singletons, especially during the third trimester. Conclusions: The fetal biometric trajectories demonstrated characterized pattern according to chorionicity and conception mode. We are the first to curve fetal biometric reference values for Chinese SC-MCDA, SC-DCDA and ART-DCDA twin pregnancies.