If you’re a mother of twins, you’ve probably had people come up to you and ask if twins run in your family. Many women have heard this before, but few are entirely certain whether or not the likelihood of giving birth to twins is truly genetic, or if it simply happens by chance. You might be surprised to find out that the notion isn’t entirely a myth. To understand the genetic forces behind having twins, you first need to understand how twins are born.

There are two types of twins a woman can have. Monozygotic twins are the ones that are identical, and their genetic build is essentially the same. These twins were formed after the mother’s egg was fertilized. The egg split into two and became two separate eggs, and eventually two separate people. Rest assured that this is simply a strange bodily occurrence, and there is nothing genetic about it.  It is nature’s cloning.

On the other hand, dizygotic twins are those that are fraternal. They might be different genders, and they look no more alike than regular siblings. These twins were actually formed when the mother released two eggs at the same time. Both eggs were fertilized separately and two people began to form. Here is where genetics come into play. If a woman has released two eggs at the same time, she is predisposed to hyperovulation. Most women release a single egg with every cycle, but women with dizygotic twins release two (or sometimes more in the case of multiples). Hyperovulation is in fact genetic. If your mother or grandmother experienced hyperovulation, you probably will too. Certain tribes in Africa are prone to multiple ovulation and consequently a high incidence of twinning.  Women of color, older women, women with several children (high parity) are more likely to have twins.  Conversely, Asian mothers are about half as likely to have dizygotic twins.

Women taking fertilization treatments using Clomid or Pergonal will be more likely to hyperovulate resulting in a dizygotic (or monozygotic) pregnancy or even higher-order multi-fetal gestations, such as Octomom.  Also, those women who are undergoing artificial reproductive technologies, such as in vitro fertilization, which may insert more than one fertilized egg into the uterus may have twins.   But these women are not genetically predisposed to carrying twins.  The bottom line is that if you or other members of your blood-related family are dizygotic twins and you’re trying to conceive, you might want to stock up on twice the amount of baby supplies.