Tvilling

Tvilling is the term used for one of two offspring born in the same pregnancy. Biologically, twins are categorized as monozygotic (identical) and dizygotic (fraternal). Monozygotic twins arise when a single fertilized egg splits into two embryos, producing two individuals with essentially identical genetic material and usually the same sex. Dizygotic twins result from two separate eggs being fertilized by two different sperm, and they are genetically no more alike than ordinary siblings; they may be the same or different sex. In Norwegian usage, tvilling is singular and tvillinger is the plural form.

The occurrence and development of twins vary by population, maternal age, and other factors. Dizygotic twinning is more common among older mothers and in certain ethnic groups, and its frequency can be increased by fertility treatments. Monozygotic twinning occurs at a relatively constant rate worldwide. In twin pregnancies, the placental arrangement can differ: dizygotic twins typically have two separate placentas and amniotic sacs (dichorionic diamniotic), while monozygotic twins can have varying arrangements depending on when the split occurs, ranging from two placentas and two sacs to one placenta with shared sacs. This has implications for prenatal monitoring and potential complications.

Twin births are associated with higher risks compared with singleton pregnancies, including greater chances of preterm birth and low birth weight, and, in some cases, complications specific to shared placental circulation in monochorionic twins. Twins often require enhanced obstetric surveillance and specialized neonatal care at birth. The presence of twins can influence family planning, healthcare considerations, and social dynamics in communities that recognize tvilling as a distinct familial category.