Do Twins Skip a Generation?

Twins

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When it comes to twinning, people have wondered for years how it occurs and what the likelihood is that they will have twins. They are especially curious about the probability of twinning if they already have a set of twins in their family lineage.

But scientists still know little about why some couples have identical twins and others do not. They do, however, know a little more about fraternal (non-identical) twins and the likelihood of them running in families. Here's a closer look at what researchers know about twins, specifically fraternal twins.

Overview

To better understand the factors contributing to twinning, it's important to know the difference between identical twins and fraternal twins. Monozygotic (MZ) twins, which are better known as identical twins, occur when a single egg is fertilized by a single sperm cell.

MZ twins occur in every 3 to 4 per 1,000 births world wide. MZ twins are thought to be random; no genetic component has been identified that increases identical twinning. However, there have been reports of families with more identical twins than expected, which does suggest genetics may play some unrecognized role.

Meanwhile, dizygotic (DZ) twins, also known as fraternal twins, occur when two eggs are fertilized by different sperm cells during the same menstrual cycle. DZ twins are more common than MZ twins, and they are much more likely to run in families.

What's more, if a woman has a mother or a sister who have had DZ twins, they are about twice as likely to have DZ twins as well.

Yet, studies that have attempted to identify the contributions of specific genes have had mixed as well as conflicting results. Few specific genes have been definitively linked to an increased probability of DZ twinning.

Do Twins Skip a Generation?

A commonly held myth about twins is that twins skip a generation. For instance, some people believe it's unlikely that a twin will have twins, but they may be more likely to have twin grandchildren.

This belief is based on the assumption that twinning is genetic and runs in families. However, if that was truly the case—if there was a twin gene—then twins would occur with predictable frequency in those families that carry the gene.

But there is no concrete scientific evidence that suggests twins skip a generation.

That being said, there are several factors that make some women more predisposed to conceiving DZ twins. The most common one is hyperovulation, which is the tendency for a woman to release more than one egg during her menstrual cycle. If two eggs are fertilized, DZ twinning can occur.

Hyperovulation and Fraternal Twins

So far, no specific genes have been definitively proven to cause hyperovulation or increase the chance of having fraternal twins. Any discussion of a gene that causes hyperovulation is only theoretical at this point.

That being said, researchers have discovered two genes that may make women more susceptible to having DZ twins. These genes are carried by women, so the daughter of a woman who carries these genes may be more likely to have twins. This may be the reason why twinning is thought to skip a generation, but until scientists uncover a concrete link, it is purely speculation.

Researchers indicate that if the cause of twinning is related to hyperovulation, only the mother's genetics would influence the chances for her to have twins, though.

The father’s role is irrelevant to the current generation. Overall, there is still more research that needs to be done to understand hyperovulation, genetics, and DZ twinning.

Interestingly, DZ twinning is a relatively common occurrence that happens roughly 13 times per 1,000 pregnancies. Keep in mind that twinning frequency varies over time and according to geographic location.

In fact, of the 30 people you meet in the U.S. or Europe, one of them is likely to have a twin. Meanwhile, the lowest chance of meeting a twin is in Asia, where 1 in 70 people is a twin. The most likely place you will meet a twin is in Nigeria, where 1 in 12 people is a twin. These variations are thought to be influenced by a number of factors, including genetics and environment.

Other Factors Contributing to Twinning

The tendency to have DZ twins is a complex trait that is not only influenced by hyperovulation but also by a number of other factors, including the mother's environment and her genetic disposition. For instance, a woman's age as well as her body composition have been loosely linked to DZ twinning.

In fact, Percy Nylander, a researcher who laid the groundwork for twinning studies, discovered that for tall women, the risk of having twins was 1.5 to 2.0 times higher than that of shorter women. Likewise, other studies have shown that twin mothers are typically taller and have a higher BMI (body mass index) when compared to mothers of singletons.

Consequently, a BMI of 30 or more was associated with an increased risk of twinning.

A mother's age and the number of children she has had also may influence DZ twinning. In fact, DZ twinning rates increase four-fold from age 15 to age 35. The increase in twinning as a mother ages is thought to be due to rising FSH hormone concentrations, which causes more ovarian follicles to mature and more eggs to be released.

Researchers have looked for other explanations for DZ twinning, including the use of oral contraceptives and increased intake of folic acid. Even seasonality and smoking may influence DZ twinning. But, at this point, the results of these studies have been inconclusive and more research is needed to determine the role these factors play.

A Word From Verywell

There is still a lot that is unknown about what causes a couple to have twins. Likewise, while there may be a specific gene that causes twinning to occur, scientists have yet to uncover it.

Until then, DZ twinning is loosely explained by a mother's age, body composition, hyperovulation, and a number of other possible factors. If you happen to be expecting twins, rest assured that scientists are working diligently to learn all they can about what causes twinning.

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