Twin refers to either of two offspring produced in the same pregnancy. In humans and many other mammals, twins result when one fertilized egg divides or when two separate eggs are fertilized at roughly the same time. Twins may be born minutes, hours or even days apart and often require specific prenatal and neonatal care because multiple pregnancies change the mother's physiology and increase certain risks.

Types and biological basis

  • Monozygotic (identical): a single fertilized egg (zygote) splits into two embryos. These twins typically share the same genetic material and are of the same sex. The timing of the split affects whether they share a placenta or amniotic sac; incomplete separation can produce conjoined twins.
  • Dizygotic (fraternal): two different eggs are fertilized by two different sperm. Genetically they are like ordinary siblings and can be same or different sexes. Dizygotic twinning is influenced by maternal biology and external factors.

Other, less common phenomena include superfetation (a second ovulation and fertilization during an ongoing pregnancy) and superfecundation (two eggs fertilized by sperm from separate acts of intercourse), which can lead to unusual genetic combinations.

Development, birth and medical considerations

Multiple gestations tend to increase the likelihood of preterm birth, lower birth weights and delivery by cesarean section. Specific twin complications include unequal growth and twin-to-twin transfusion syndrome in monochorionic pregnancies (when twins share a placenta). Prenatal monitoring, specialized obstetric care and neonatal support are commonly recommended to manage these risks.

Research, genetics and social aspects

Identical twins have been central to studies of heredity and environment because their near-identical genomes provide a powerful natural comparison. Twin registries and longitudinal studies have informed knowledge on behavior, disease risk and development. Culturally, twin births have attracted attention in traditions and folklore, sometimes celebrated and sometimes treated with superstition.

In everyday terms, the main distinctions to know are "identical" (one egg split) and "fraternal" (two eggs). Medical advances and assisted reproductive technologies have changed twin incidence patterns in recent decades. Despite shared beginnings, twins can differ widely in personality, health and life course, illustrating the interplay of genetics and environment.