Do Twins Run In Families? Here's When Genetics Play A Role, & When They Don't

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Twins have always been a subject of fascination, and you probably wonder about the odds of having them if you're expecting. In fact, thinking about whether twins run in families is probably a rite of passage for most moms-to-be, especially those related to twins. After all, to anyone who grew up watching Sister, Sister or The Parent Trap, having a twin looks like an absolute blast.

For starters, the answer depends on the type of twin. "Genetics can definitely play a role in having fraternal twins," said Dr. Anja Scholze of Stanford University in The Tech Museum of Innovation. "For example, a woman that has a sibling that is a fraternal twin is 2.5 times more likely to have twins than average!" The exact reason for this increased likelihood is still being studied. But it looks like two genes that affect hormone levels may be responsible for the increased likelihood of fraternal twins in some families, according to a 2016 study in The American Journal of Human Genetics. Like so much else, it all comes down to what's in your exact genetic makeup.

As a quick recap, fraternal twins occur when two different fertilized eggs develop simultaneously, according to the American Academy of Pediatrics (AAP). They are essentially siblings who happen to live in the womb at the same time, and their DNA and appearance will be as similar as any other pair of siblings. That's why you might know some twins who look nothing alike.

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So if fraternal twins are more likely to occur in certain families, why not identical twins? It's all about the way the different types of twins develop. Unlike fraternal twins, identical twins are formed when the same fertilized egg splits in two, according to the U.S. Office on Women's Health. Because identical twins are relatively rare and occur randomly across the population, there does not appear to be a genetic predisposition for them, as noted by What To Expect. They are just a big surprise. Even if a relative has an identical twin, your odds of giving birth to identical twins are about the same as everyone else's. Weird, right?

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If you're super into the idea of multiples, can you do anything to make yourself have twins? Not exactly, but certain other factors surrounding a pregnancy may affect the odds of giving birth to fraternal twins. (Again, identical twins just kind of randomly happen.) Women with a greater number of previous pregnancies, as well as those in their 30s and 40s, have a greater likelihood of having twins, according to Better Health Channel (of the Department of Health & Human Services, State Government of Victoria, Australia). In addition, using assisted reproductive techniques, which often release several eggs at once, can increase the odds of having two (or even more) babies at once, as further explained by the Better Health Channel. Basically, having several children, as well as continuing to have kids later in life, may increase the chances of having fraternal twins. But all that being said, there are no guarantees when it comes to babies.

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For most families, twins of any sort are something of a rarity. The odds of having an identical twin are about three in 1,000, according to Very Well. Plus, the prevalence of identical twins are pretty uniform around the world. In addition to the previously mentioned factors including maternal age and number of previous births, the frequency of fraternal twins varies depending on the country and population. For example, African-American women are the most likely to have twins of any race, whereas Asian and Native American women are least likely to have twins, according to Johns Hopkins Medicine. The particular reasons for these differences are still being studied, but one fact remains: having twins is a rare and special event for any family.

Check out Romper's new video series, Bearing The Motherload, where disagreeing parents from different sides of an issue sit down with a mediator and talk about how to support (and not judge) each other’s parenting perspectives. New episodes air Mondays on Facebook.