Family Tips

Multiple pregnancies in most cases are not casual and there are several factors which may influence it:
The first and the main one is heredity: multiple births on a woman’s side of the family increases her chances to have twins.
Race is also important. It appeared that women of African descent are the most likely to have multiple pregnancies.
Number of prior pregnancies. If a woman had more than one previous pregnancy, especially a multiple pregnancy, the chances to have a multiple pregnancy encrese.
Delayed procreation. Older women who are pregnant more likely will have multiples.
Barrenness treatment. Fertility drugs, which stimulate the ovaries to produce multiple eggs, or assisted reproductive technology (ART), which transfers multiple embryos into the womb (such as in vitro fertilization and embryo transfer, IVF & ET), greatly increase chances to have twins or even triplets. 
The last two factors are probably responsible for the increase in multiple births in the last couple of years.