Do twins run in the family?

Roberta Torricelli

This is a very common question among people and in a way yes, they do, but the answer is far more complex than that.

Twins are relatively common and on average, out of 1000 pregnancies, 13 will be twins, although the incidence varies over time and geographic location. This variation is mostly attributed to the differences in fraternal (dizygotic) twinning frequency, as the incidence of identical (monozygotic) tends to remain constant. Genetics has been shown to play a role into this, particularly in one’s chance of having dizygotic twins. As a matter of fact, the chance of having fraternal twins is approximately two times greater for women whose mother or sister(s) had previous fraternal twins. The difference between the two types of twin pregnancies and frequencies lies in the modality of fertilisation: monozygotic twins result from the fusion of one sperm with one egg, whereas dizygotic twins are formed by two different sperm cells fertilising two separate eggs.

The increase in chance of dizygotic twin pregnancy following family history, is suggested to be due to the presence of a gene that can promote hyperovulation (the release of multiple eggs in the same cycle) informally identified as the “twin gene.” However at the moment, this is just a hypothesis as no specific gene has been identified yet to hold such a specific function. Furthermore, it is important to note that as it is women who ovulate, male can only be “carriers” of this supposed gene, and do not have an increased chance to generate a twin pregnancy. Instead, they might pass it onto their daughters which in return, might have a higher chance of having twins themselves. This phenomenon could account for the “twins skip a generation” belief. However, because there aren’t any definitive links between genes, hyperovulation and twin pregnancy, it is likely that when twins do skip a generation, it may simply be due to coincidence.

Finally, it’s important to understand that there may be other factors besides genes that increase a woman’s chance of having twins. Fertility treatments, such as in vitro fertilization (IVF), BMI, age and ethnicity, can all increase the incidence of multiple pregnancies. For this reason, the incidence of twin pregnancies, and in particular of fraternal twin pregnancies, is far more complex to predict than expected. Therefore, although certain genes may run in families to increase the odds of a fraternal twin gestation, it is likely that other variables will factor into it as well.