The exact cause of twinning is not known. The frequency of monozygotic twins remains constant through the globe and is probably related to maternal environmental factors. It is the wide variation in the prevalence of dizygotic twins which is responsible for the fluctuation in the overall incidence of twins in different populations. Possible causes are following :
- Superfetation is the fertilization of two ova released in different menstrual cycles. The nidation and development of one fetus over another fetus is theoretically possible until the decidual space is obliterated by 12 weeks of pregnancy.
