Chinese Population Reference Curves for Ultrasound-Measured Amniotic Fluid Deepest Vertical Pocket in Dichorionic Twin Pregnancies, and Their Associations With Pregnancy Outcomes

Objective: To establish the Chinese population's amniotic fluid deepest vertical pocket (DVP) reference curves for dichorionic twin pregnancies and to investigate DVP links with adverse perinatal outcomes. Methods: This retrospective cohort study, conducted at Peking University Third Hospital from August 2011 to December 2020, used data from 375 women aged 20 to 45 years who had dichorionic twin pregnancies that were confirmed through first-trimester ultrasound. After exclusions, reference curves were developed using 318 women with 3,299 DVP scans, and the data analyzed included maternal demographics, pregnancy outcomes, ultrasound measurements, and neonatal information. DVPs were assessed via ultrasound at regular intervals and grouped by gestational age (GA) for analyses. Linear mixed models were used to create amniotic fluid reference curves. Associations between abnormalities and adverse perinatal outcomes were examined using Chi-squared or Fisher's exact tests. Logistic regression provided both crude and adjusted odds ratios, adjusting for pre-pregnancy weight, age, ethnicity, parity, and conception mode. Significance was set at P < 0.05 with 95% confidence intervals (CI), and the analyses were conducted using SPSS ver. 26.0 (IBM Corp., Armonk, NY) and SAS ver. 9.4 (SAS Institute Inc., Cary, NC, USA) software. Results: DVP increased from the first trimester to a maximum at 26 weeks (95% confidence interval (CI), 2.7-8.3 cm for twin 1 and 2.8-7.9 cm for twin 2) and then decreased gradually toward term. Differences between twins 1 and 2 were significant after 26 weeks (95% CI for twin 1:5.3, 5.5; 95% CI for twin 2: 4.4, 4.5; P < 0.010). Polyhydramnios of twin 1 increased the risk of large for GA. Oligohydramnios of twin 1 increased the risk of small for GA. Polyhydramnios of twin 2 increased the risk of small for GA, premature birth, and neonatal complications. Oligohydramnios of twin 2 increased the risk of preeclampsia, hypertensive disorder complicating pregnancy, and premature rupture of membranes. Conclusion: Reference curves for twin amniotic fluid volumes vary by GA and differ between twins, with potential implications for pregnancy outcomes.