People are very curious about a family or genetic connection to twinning. I often get emails inquiring, "My grandmother's uncle was a twin. Will I have twins?" And when I tell people that I have twins, one of the first questions I'm asked is, "Do twins run in your family?," almost as if they aim to justify or explain my daughters existence as part of a historical pattern.
Do Twins Run in Families?
There are plenty of families with multiple sets of multiples, for example, the Davis family of Frebericksburg, Virginia who have three sets of twins, or Eric and Elizabeth Hayes of New Jersey, parents of two sets of twins and sextuplets. There are also families who seem to have many sets of twins spread among generations, twin cousins and grandparents and nieces and nephews.
But do twins really run in families? Actually that's probably one of the most widely perpetuated myths about multiples. While some families do seem to have a preponderance of multiple births, it's often more than a coincidence than a connection.
Is There a "Twin Gene"?
Of the factors that influence multiple birth, there is only one that could be identified as genetic or explained by family history: hyper ovulation. Hyper ovulation is the tendency to release multiple eggs during ovulation, increasing the chances of conceiving dizygotic (or fraternal) twins. So, in families where the women have a gene for hyperovulation, genetics would sufficiently explain an increased presence of fraternal twins.
However, only women ovulate. So the connection is only valid on the mother's side of the family. While men can carry the gene and pass it on to their daughters, a family history of twins doesn't make them any more likely to have twins themselves.
Fraternal twins "run in families" on the mother's side only, if she inherits the gene for hyper ovulation.
Do Twins Skip a Generation?
If your father was a twin, but you weren't, are you more likely to have twins? It's a common misconception that twins skip a generation in families. There is absolutely no evidence, other than circumstantial, that twins are more likely to occur every other generation. However, if you consider the influence of genetic hyper ovulation, this pattern could appear in families depending on whether their children were sons or daughters.
Generation 1: Grandma
Grandma has the gene for hyperovulation. She & Grandpa have fraternal boy twins, Rob and Bob
Generation 2: Rob & Bob
While Grandma's sons may carry the gene for hyperovulation, they do not ovulate. They will not have twins (excluding other factors). However, they each have a daughter.
Generation 3: Molly & Polly
Molly and Polly, cousins, inherit the gene for hyperovulation from their fathers. They each have a set of twins.
You can see how this example makes it appear that twins skip a generation in families. The pattern is influenced by whether the inheritor of the hyper ovulation gene is male or female. For more information, consult this article: Do Twins Skip a Generation?
What About Identical Twins?Although theories and research abounds, there is no established connection between genetics and monozygotic (identical) twinning. Scientists haven't really identified and confirmed a clear cause for monozygotic twinning, which occurs when a fertilized egg splits and develops into two (or more) embryos. About ten years ago, I read about a theory that suggested that there was an enzyme in sperm that could cause an egg to split after fertilization, however I have not encountered any conclusions from that research. At this time, monozygotic twinning appears to be a random event, and all parents have a reasonably equal chance of conceiving identical twins.
Identical twins are random and do not run in families, except by coincidence.