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.
- Superfecundation is the fertilization of two different ova released in the same cycle, by separate acts of coitus within a short periods of time.
- Iatrogenic causes - Drugs used for induction of ovulation may produce multiple fetuses to the extent of 20-40% following gonadotrophin therapy.
- Influence of parity - The incidence is increased with increasing parity specially from 5th gravida onwards.
- Advancing age of the mother - Twinning increase with maternal age 30-35 years.
- Race - The frequency is highest among as Negroes, lowest among as Mangols.
- Hereditary.
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