Do Twins Run in Families?

Releasing more than one egg per month is the genetic culprit

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People are very curious about a family or genetic connection to twinning as twins are a fascinating phenomenon. 

If you suspect a genetic link, you are partially right, but the link is quite complex. For one, genes may play a role in your chance of having fraternal twins (called dizygotic twins) but not identical twins (monozygotic twins). Secondly, it's important to understand that there may be other factors besides genes that increase a woman's chance of having twins.

Is There a "Twin Gene"?

According to genetic research, the chance of having fraternal twins is approximately two times greater for women whose mother or sister had fraternal twins. This increased chance is due to a gene that promotes hyperovulation—sometimes referred to as the "twin gene."

Hyperovulation is the tendency to release more than one egg during ovulation, which increases the chances of conceiving dizygotic (or fraternal) twins. So, in families where the women have a tendency towards hyperovulation, genetics could sufficiently explain an increased presence of fraternal twins.

However, since only women ovulate, 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. But, if a father passes on the "twin gene" to his daughter, then she may have a higher chance than normal of having fraternal twins.

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 hyperovulation, 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 and 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 hyperovulation gene is male or female. 

Is There a Link Between Identical Twins and Genetics?

Although theories and research abound, there is no established connection between genetics and monozygotic (identical) twinning. Scientists haven't really identified a clear cause for monozygotic twinning, which occurs when a fertilized egg splits and develops into two (or more) embryos.

At this time, monozygotic twinning appears to be a random event, so all parents have a reasonably equal chance of conceiving identical twins.

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

There is no way to predict precisely who will and who will not have twins.

That being said, when calculating your chances, consider these factors: 

What Kind of Twins Are in the Family?

Remember, monozygotic (identical) twins don't run in families—they are random. You may not know if your great-granduncles were identical or not and often there is no way to know for sure without DNA testing. That being said, twins that share a close physical resemblance are more likely to be identical than fraternal. Also, remember that boy/girl twins are always fraternal (dizygotic).

Are the Twins a Result of Assisted Reproduction?
In the last 10 years, a large number of twin and multiple births are attributed to fertility treatments including fertility drugs and in vitro fertilizations.

For example, the fertility drug Clomid can stimulate the release of more than one egg during ovulation. This can lead to fraternal twins if two eggs are fertilized. 

Whose Side of the Family Had 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, you might have the gene. But your husband's family history has absolutely no bearing on your own children, except maybe for the future possibility that you might have twin grandchildren—if you have a daughter who inherits the hyperovulation gene.

A Word From Verywell

Remember, a family history of twins is only one of many factors that influence multiple births. Maternal age, race, weight, diet, and reproductive history all contribute to twinning and may have a stronger influence than family history.

Sources:

American College of Obstetrics and Gynecologists. (July 2015). Frequently Asked Questions: Multiple Pregnancy.

Online Mendelian Inheritance in Man (OMIM): An Online Catalog of Human Genes and Genetic Disorders. (June 2016).

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