Genetics meets embryology: redefining zygosity in a dichorionic diamniotic twin pregnancy through STR profiling

Introduction Twin pregnancies carry high maternal and neonatal risks and are clinically classified by chorionicity and amnionicity. One type of twin pregnancy is dichorionic diamniotic (DCDA) twins, in which each fetus has its own chorion and placenta. Most DCDA twins are dizygotic (DZ); however, a small percentage are monozygotic (MZ). Therefore, placental number alone cannot reliably determine zygosity. Furthermore, early gestational age discrepancies between twins may raise concern for selective fetal growth restriction, asynchronous conception, or rare phenomena such as superfetation. Materials and Methods A DCDA twin pregnancy was evaluated after ultrasonography, which demonstrated a 7–9 days discrepancy in estimated gestational age between fetuses. Postnatal short tandem repeat (STR) profiling was performed using standard forensic genetic methodology to determine zygosity. Results Despite the fetal growth discordance, serial ultrasound examinations showed parallel growth trajectories without clinically significant biometric discordance. Delivery occurred by cesarean section with favorable neonatal outcomes and highly similar anthropometric measurements in both newborns. Complete concordance across all analyzed STR loci was observed, strongly supporting MZ origin and indicating a dichorionic diamniotic MZ twin pregnancy. Discussion This case illustrates that chorionicity alone does not determine zygosity and that early gestational-age discrepancies should be interpreted cautiously, as their underlying cause may not be ascertainable from ultrasonographic findings alone. STR-based testing may provide valuable information in selected twin pregnancies with discordant clinical findings.