Increased fetal fraction is associated with the incidence of birth weight discordance and selective fetal growth restriction：a retrospective cohort study

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.