Association of zygosity and chorionicity with anatomical distribution of congenital malformations in twin pregnancies.

objectiveTo investigate the incidence, anatomical distribution, severity and concordance of congenital malformations (CMs) in twin pregnancies according to both zygosity and chorionicity in a cohort selected to minimize non-embryological confounding factors.

methodsThis was a retrospective single-center cohort study including twin pregnancies evaluated at a fetal medicine referral center (Fetal Medicine Unit, Di Venere Hospital, Bari, Italy) between 1 January 2016 and 31 December 2025. Pregnancies conceived via assisted reproductive technology and those complicated by major maternal metabolic or systemic conditions or abnormalities attributable to secondary hemodynamic complications of monochorionic pregnancy were excluded. Chorionicity was determined on first-trimester ultrasound. Zygosity was established by invasive prenatal molecular testing when available or by standard clinical criteria; molecular confirmation was obtained for almost all pregnancies with structural CMs. CMs were classified according to anatomical district, severity (major vs minor) and concordance. Analyses were performed at pregnancy level. Based on descriptive evidence of anatomical clustering, logistic regression analysis was performed using monozygosity as the exposure variable and cardiac and/or central nervous system (CNS) malformations as the outcome.

resultsAmong 936 twin pregnancies evaluated, 564 met the inclusion criteria. Overall, CMs were identified in 102 (18.1%) pregnancies, including 57 (55.9%) major and 45 (44.1%) minor anomalies. The overall prevalence of CMs was broadly comparable between twin types. CMs were predominantly discordant between cotwins across all groups. Monozygotic pregnancies demonstrated a distinct non-random anatomical distribution pattern characterized by enrichment of cardiac anomalies, particularly conotruncal defects, and CNS anomalies, particularly neural tube defects. When pregnancies were analyzed according to zygosity alone, cardiac and CNS anomalies accounted for over half of major CMs in monozygotic twins compared with approximately one-quarter in dizygotic twins. Logistic regression analysis confirmed an association between monozygosity and cardiac and/or CNS malformations (odds ratio, 4.13 (95% CI, 1.02-16.68); P = 0.047).

conclusionsMonozygosity was not associated with an increased overall prevalence of CMs but with selective anatomical clustering and a predominance of discordant phenotypes. These findings support the concept that monozygotic twinning may influence early developmental patterning rather than simply increasing the overall risk of malformation. © 2026 International Society of Ultrasound in Obstetrics and Gynecology.