Pamela Prindle Fierro is the author of number of parenting books and also the mommy of pair girls." data-inline-tooltip="true">Pamela Prindle Fierro


When it pertains to twinning, civilization have wondered because that years exactly how it occurs and also what the likelihood is the they will have actually twins. They can be especially curious around the probability that twinning if they already have a collection of pair in their household lineage.

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


To far better understand the factors contributing to twinning, it"s important to recognize the difference between identical twins and fraternal twins. Monozygotic (MZ) twins, i beg your pardon are far better known as identical twins, take place when a single egg is fertilized by a single sperm cell.

MZ twins take place in every 3 come 4 every 1,000 births worldwide. MZ twins space thought to it is in random; no genetic component has actually been identified that increases the same twinning. However, there have been reports of families with much more identical twins 보다 expected, which go suggest genetics may play part unrecognized role.

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

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

Yet, studies that have attempted to identify the contribute of details genes have had actually mixed and conflicting results. Few specific genes have been definitively attached to an increased probability of DZ twinning.

do Twins Skip a Generation?

A frequently held notion around twins is the they skip a generation. For instance, part people think it's unlikely that a twin will have actually twins, yet they might be an ext likely to have twin grandchildren.

This belief is based upon the presumption that twinning is genetic and also runs in families. However, if the was important the case—if there to be a twin gene—then twins would take place with predictable frequency in those families that lug the gene.

That gift said, there are several components that make some women much more predisposed to conceiving DZ twins. The most typical one is hyperovulation, i beg your pardon is the propensity for a mrs to release an ext than one egg throughout her expression cycle. If 2 eggs room fertilized, DZ twinning deserve to occur.

Hyperovulation and Fraternal twins

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

That gift said, researcher have uncovered two gene that might make women much more susceptible to having actually DZ twins. These genes are carried by women, for this reason the daughter the a woman who carries this genes might be much more likely to have twins. This might be the reason why twinning is assumed to skip a generation, but until scientists uncover a concrete link, that is purely speculation.

Researchers show that if the reason of twinning is regarded hyperovulation, just the mother"s genes would affect the possibilities for she to have actually twins, though.

The father’s role is irregularity to the existing generation. Overall, over there is still much more research that needs to be done to recognize hyperovulation, genetics, and DZ twinning.

Interestingly, DZ twinning is a fairly common occurrence that happens around 13 times per 1,000 pregnancies. save in mind the twinning frequency varies end time and according to geographical location.

In fact, that the 30 world you fulfill in the U.S. Or Europe, one of them is most likely to have actually a twin. Meanwhile, the lowest possibility of meeting a twin is in Asia, wherein 1 in 70 human being is a twin. The most most likely place girlfriend will meet a twin is in Nigeria, wherein 1 in 12 people is a twin. these variations room thought to be influenced by a variety of factors, consisting of genetics and environment.

Other determinants Contributing to Twinning

The tendency to have actually DZ pair is a facility trait the is no only influenced by hyperovulation but also by a variety of other factors, including the mother's environment and also her genetic disposition. Because that instance, a woman's age and her body composition have actually been loosely attached to DZ twinning.

In fact, Percy Nylander, a researcher that laid the groundwork for twinning studies, uncovered that because that tall women, the hazard of having actually twins to be 1.5 to 2.0 times greater than that of shorter women. Likewise, various other studies have shown that twin mother are frequently taller and have a greater BMI (body massive index) when contrasted to mother of singletons.

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

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

A Word from Verywell

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

Until then, DZ twinning is loosely described by a mother's age, human body composition, hyperovulation, and also a number of other possible factors. If you take place to be expecting twins, rest assured that researchers are working diligently to find out all they can around what causes twinning.

Verywell family uses just high-quality sources, consisting of peer-reviewed studies, to assistance the truth within our articles. Review our editorial procedure to learn much more about just how we fact-check and keep our contents accurate, reliable, and trustworthy.

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