OP16.06: Association between birth outcomes and zygosity in dichorionic diamniotic twin pregnancies

This study aimed to investigate the relationship between birth outcomes and zygosity in dichorionic diamniotic (DCDA) twin pregnancies and assess the relationship between the discordance of fetal fraction (FF) in cell-free DNA (cfDNA) and birthweight in dizygotic DCDA twins. Pregnant women with DCDA twins between 11+0 and 13+6 gestational weeks managed at five hospitals between May 2021 and May 2023 were registered. Blood sampling was performed to assess zygosity and FF in cfDNA using PanoramaTM (Natera, Inc.). Data on maternal background and birth outcomes were collected prospectively. The calculation for the discordance rate of FFs and birthweight is as follows: (larger value - smaller value) / larger value. In 117 pregnant women with DCDA twins, the median maternal age, height, and body weight before pregnancy were 34 years, 160cm, and 52kg, respectively. There were 77 (66%) primiparas and 33 (28%) pregnancies with assisted reproductive technology. The median gestational age (GA) at blood sampling was 12+4 weeks. Dizygotic (DZ) twins, monozygotic (MZ) twins, and no results accounted for 98, 17, and 2 cases, respectively. In 234 fetuses, the number of males, females, and no results for fetal sex were 124, 96, and 14, respectively. The average GA at delivery was 37+0 weeks. Although maternal age, primipara, fetal sex, and average GA at delivery showed no differences between DZ and MZ twins, the birthweight of MZ twins was lower than that of DZ twins (2039 ± 432 vs. 2297 ± 385, p = 0.01). The average FF in cfDNA and the discordance rate of FFs in DZ twins were 7.4% and 13.5%, respectively. The average discordance rate of birthweight in DZ twins was 10.6%. The discordance rate of FFs in cfDNA in DZ twins was not associated with birthweight discordance (r = 0.01). The birthweight of MZ twins was lower than that of DZ twins. The discordance of FFs in cfDNA in DZ twins was not associated with birthweight discordance.