- Preterm labor and birth.
- Gestational high blood pressure.
- Gestational diabetes.
- More than 3 in 5 twins and nearly all higher-order multiples are premature (born before 37 weeks).
The frequency of N multiple births from natural pregnancies has been given as approximately 1:89 (Hellin's law) and as about 1:80. It is preferable to avoid the risk of higher-order multifetal pregnancy by limiting the number of embryos to be transferred or by cancelling a gonadotropin cycle when the ovarian response suggests a high risk of a multifetal pregnancy.
- Heredity. A family history of multiple pregnancy raises the chances of having twins.
- Older age. Women older than 30 have a greater chance of multiple conception.
- High parity.
- Race
