Twins

Twins are two individuals produced during a single pregnancy. They can be categorized as monozygotic (identical) or dizygotic (fraternal). Monozygotic twins originate from a single fertilized egg that splits into two embryos; they share the same genetic material. Dizygotic twins arise from two separate eggs fertilized by two sperm; they are genetically similar to ordinary siblings and may be of the same or different sexes. In monozygotic pregnancies the twins may share a placenta or amniotic sac, depending on when the split occurs, while dizygotic twins almost always have separate placentas and amniotic sacs.

Incidence varies: monozygotic twinning occurs at a relatively constant rate worldwide, about 3 to 4 per 1,000 births. Dizygotic twinning rates vary by ethnicity, maternal age, fertility treatments, and other factors, and can range from roughly 8 to 16 per 1,000 births in some populations.

Prenatal pregnancy and care for twins involve higher risks and specialized monitoring. Twins are at greater risk for preterm birth, low birth weight, and complications such as twin-to-twin transfusion syndrome in monochorionic pregnancies. Routine prenatal care includes more frequent checkups and ultrasound to determine chorionicity and fetal growth. Genetic testing may be offered when indicated, and management plans are tailored to the pregnancy.

Impact and study: Twins have contributed to medical and behavioral genetics through twin studies that compare similarities between monozygotic and dizygotic pairs to estimate heritability. Culturally, twinning rates and societal attitudes toward twins vary by region and history. Notable twin pairs have appeared across literature and media, illustrating the diversity of twin experiences.