(1194) Impact of Zygosity on Perinatal Outcomes in Dichorionic Diamniotic Twin Pregnancies

Objective:
Single Nucleotide Polymorphism (SNP) Noninvasive prenatal testing (NIPT) allows determination of zygosity in twin pregnancies. Prior studies used NIPT to explore associations between monozygosity and adverse neonatal outcomes regardless of chorionicity. Our aim was to compare perinatal outcomes between SNP based NIPT-confirmed monozygotic and dizygotic twins in only dichorionic diamniotic (DCDA) twin pregnancies.

Study Design:
We conducted a single-center, IRB-approved retrospective cohort study of DCDA twin pregnancies with NIPT results between 2021 and 2025. Patients were excluded if data were missing for zygosity or outcomes of interest. Baseline characteristics were compared by zygosity using t-tests or chi-square/Fisher’s exact tests, as appropriate. Logistic regression models estimated the association between zygosity and perinatal outcomes, with odds ratios adjusted for maternal age and mode of conception.

Results:
Of the 60 patients included, 10 had monozygotic pregnancies (16.7%) and 50 had dizygotic pregnancies (83.3%). The odds of congenital anomaly were 7.90 times higher in monozygotic pregnancies compared to dizygotic pregnancies (95% CI, 1.43–43.55). The odds of genetic anomaly were 18.86 times higher in monozygotic pregnancies (95% CI, 1.56–228.46). No neonatal deaths or miscarriages were observed in either group. There were no statistically significant differences in rates of preterm birth, NICU admission, or other perinatal outcomes.

Conclusion:
Monozygotic DCDA twin pregnancies were associated with significantly higher odds of congenital and genetic anomalies compared to dizygotic twins. It is unclear at this time whether monozygotic twinning increases the risk of genetic anomalies or whether preexisting genetic abnormalities may predispose embryos to monozygotic splitting. These findings underscore the importance of utilization of SNP-based NIPT testing to determine zygosity of DCDA twin pregnancies. Monozygotic DCDA twins may benefit from the routine offer of diagnostic testing and detailed anatomic screening.