There is no evidence that the probability of having identical twins is determined by genetics. However, the likelihood of conceiving non-identical twins is determined by genetics. A couple is more likely to get twins if there are twins in the woman’s family.
Identical twins occur when one fertilised embryo breaks into two. Sometimes, identical twins are referred to as monozygotic twins. This results to each baby having the same genes. Therefore, they look very alike and they have same sex–both girls and both boys. Worldwide, monozygotic twins occur in about 3 per 1,000 births. Research suggests that most cases of identical twinning are not caused by genetic factors.
Non-identical or dizygotic (DZ) twins happen when two sperms fertilize two eggs at the same time. Non- twins are sometimes referred to as fraternal twins. Some women are genetically more likely to produce two or more eggs during the menstrual cycle. This makes them more likely to have twins.
There are also some non-genetic factors that make non-identical twins more likely. These factors include ethnic group, the age of the mother and infertility treatment.