tvillinger

Tvillinger is the Danish term for two children born from the same pregnancy. They can be categorized as monozygotic (identiske tvillinger) or dizygotic (toægget tvillinger). Monozygotic twins arise when one fertilized egg splits into two embryos, leading to twins who share nearly identical genetic material. The timing of the split can affect placenta and amniotic sac arrangements, but most monozygotic twins share a placenta. Dizygotic twins develop from two separate eggs fertilized by two different sperm and are genetically as similar as ordinary siblings; they can be the same sex or different sexes and typically have separate placentas and amniotic sacs.

Incidence and determinants: The rate of twinning varies by population and maternal factors. Monozygotic twinning occurs at a relatively constant rate worldwide, about 3–4 per 1,000 births. Dizygotic twinning shows greater variation, with higher rates in some African populations and lower rates in East Asian populations. Maternal age, parity, genetics, and family history can influence the likelihood of fraternal twinning. Fertility treatments and maternal older age have contributed to higher twinning rates in many regions.

Medical and social considerations: Twin pregnancies carry higher risk for complications such as preterm birth, growth discordance, and twin-to-twin transfusion syndrome, and thus require closer prenatal monitoring and specialized perinatal care. Most twins reach healthy outcomes with appropriate medical care, but the overall risk profile is higher than for singleton pregnancies.

Terminology: In Danish usage, tvillinger is the plural for twins; a single twin is a tvilling. The terms enkeltægget tvilling and toægget tvilling are used for monozygotic and dizygotic twins, respectively.