Do Twins Run in Families? Understanding the connection between twinning and fami

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.

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.

If I Have Twins in My Family, Will I Have Twins?

I don't have a crystal ball to know whether someone will have twins or not. If I did, I'd use it to play lottery numbers! If you have a family history of twins, and wonder whether you will have twins, consider these factors:

What kind of twins are in the family?

Remember, monozygotic (identical) twins don't run in families; they're random. You may not know if your great-granduncles were identical or not; often there is no way to know for sure without DNA testing. However, twins that share a close physical resemblance are more likely to be monozyotic. Boy/girl twins are always fraternal (dizygotic).

Are the twins a result of assisted reproduction?

In the last ten years, a large number of twin and multiple births can be attributed to fertility treatments: fertility drugs and invitro procedures. If your cousin had twins after taking Clomid, it has absolutely no impact on your chances of having twins.

Whose side of the family had twins?

Let me talk to the women for a moment, since they are the ones who usually ask me if they'll have twins. If the twins are present on your husband's/partner's side, it won't influence your chances of having twins. Remember, the gene for hyperovulation is only a factor for the mother. If your mother -- or your grandmother or aunt -- was or had fraternal twins, then you might have the gene. But your husband's family history has absoultely no bearing on your own children, except maybe for the future possibility that you might have twin grandchildren!

What other factors are at play?

Remember, a family history of twins is only one of many factors that influence multiple birth. Maternal age, race, weight, diet and reproductive history all contribute to twinning and may have a stronger influence than family history. Consult my resources on How to Have Twins to understand these other factors.