2026, Vol. 9, Issue 1, Part A

Twins refer to two embryos developing within the same pregnancy, originating either from a single fertilized oocyte (monozygotic twins) or from two separate oocytes fertilized by different sperms (dizygotic twins). Monozygotic twins, commonly termed identical twins, possess identical genetic material and are always of the same sex, whereas dizygotic or fraternal twins may be of the same or different sex due to genetic dissimilarity. Globally, twin pregnancies occur in approximately 1 out of every 80 live births, with marked regional variations. Higher rates are observed in African populations, followed by Europe and the United States, while lower incidences are reported in Asia and India. The prevalence of dizygotic twinning has increased significantly since the 1980s, largely attributable to the widespread use of fertility drugs and assisted reproductive techniques such as in vitro fertilization (IVF). Twin pregnancies are associated with a higher incidence of obstetric complications, including preterm birth, intrauterine growth restriction, twin-to-twin transfusion syndrome, congenital anomalies, and increased perinatal mortality when compared with singleton pregnancies. Placental morphology and fetal membranes play a crucial role in classifying twin types and determining clinical outcomes. Dizygotic twins typically develop with separate placentas and membranes, whereas monozygotic twins may share placental structures depending on the timing of embryonic division. Understanding the embryological basis, placental configurations, and epidemiological patterns of twin pregnancies is essential for accurate diagnosis, risk assessment, and effective clinical management of multiple gestations.