The inherited trait is releasing extra eggs
Fraternal twins begin when two separate eggs are fertilized in the same cycle. Some women are more likely to release multiple eggs at once, a tendency often called hyperovulation. If both eggs are fertilized and develop, the result can be fraternal twins—siblings who share a pregnancy but are no more genetically alike than other siblings.
Because egg release is part of the mother’s reproductive biology, the family pattern often becomes visible through women on the mother’s side. But inheritance is not limited to that route: a man can carry and pass along relevant genetic tendencies to a daughter, even though they do not change his own ovulation.
Why twins do not pass down as a package
The inherited likelihood is a predisposition, not a guarantee. A woman with relatives who had fraternal twins may never release two eggs in a cycle; the trait simply makes that outcome more likely. And having twins once does not mean every later pregnancy will be a twin pregnancy.
This pattern applies to fraternal twins, not identical twins. Identical twins form when one fertilized egg splits into two embryos, a different process. So a family history of twins can offer a clue about egg release—but it does not mean twins themselves are inherited as a fixed, repeating outcome.
