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Disclaimer: This article is for informational purposes only and is not intended to diagnose any conditions. LifeDNA does not provide diagnostic services for any conditions mentioned in this or any other article.
Many parents dream of having twins, and many of us wonder whether our genetics plays a role. Is the likelihood of giving birth to twins written in your DNA?Â
Over the past 40 years, twin births have increased by more than 30% globally. The more we understand about the hereditary likelihood of having twins, the better we can support families medically, socially, and emotionally.
There are two main types of twins, identical, called monozygotic, and fraternal, called dizygotic twins. Identical twins happen when a single egg is fertilized by a single sperm cell and then splits into two embryos during early development. These twins share the same genetic material and are always the same sex.
Fraternal twins, on the other hand, result from two separate eggs being fertilized by two different sperm cells in the same cycle. They are genetically just like regular siblings and may be the same or different sexes.
Twins are becoming more common than they used to be. Between 1980 and 2018, the number of twin births in the United States increased by about 72%. By 2018, around 33 sets of twins were born for every 1,000 births.
Naturally, about 1 in every 250 pregnancies results in twins. There are two main ways twins can be conceived: either from one fertilized egg that splits into two (identical twins), or from two separate eggs fertilized by two different sperm cells (fraternal twins). Identical twins are less common, occurring in only about 3 or 4 out of every 1,000 births. They are always the same sex and share exactly the same DNA, which is why they often look so alike.
Propensity to have identical twins is not usually influenced by genetics. The splitting of the fertilized egg appears to happen randomly, and research shows that most cases of identical twinning do not run in families. However, there have been rare reports of families with multiple cases of identical twins. Scientists suspect that genes involved in cell adhesion, the way cells stick together, might play a role in these unusual cases, but this has not been confirmed. Overall, anyone can have identical twins, and it’s not something that can be predicted or inherited.
However, a study has shown that taller women may be more likely to have fraternal twins, and this could be linked to a natural hormone in the body called insulin-like growth factor, or IGF. IGF is a protein that helps the body grow and develop. It plays a key role in childhood growth and continues to support cell and tissue repair throughout life.
Researchers looked at a group of women who had naturally conceived twins or triplets. On average, these women were about 3 centimeters taller than women in the general population. The study found that this height difference was not random, it was statistically significant, suggesting a real connection. Taller women often have more IGF, which may cause the ovaries to release more than one egg at a time. This process, known as hyperovulation, is important in the conception of fraternal twins, when two eggs are fertilized in the same cycle. In short, the study supports the idea that a woman’s height and the hormones linked to it could slightly raise the chances of having twins. While height alone doesn’t determine twin pregnancies, it may be one small part of the bigger picture.
Other studies have also shown that certain genes might be linked to the chance of having fraternal twins, but the results weren’t clear-cut. In 2010, researchers looked at people from different parts of the world and found that a region around geneFAM120B on chromosome 6 might play a role in twinning. However, differences between genetic regions suggested that more than one gene could be involved. Another study in 2016 found that genetic variants in FSHB and SMAD3, both related to hormone function—may also increase the chances of having twins. This supports the idea that twinning is likely influenced by many genes, especially those involved in hormones and ovulation.Â
A recent large-scale GWAS study shares insights into the genetic basis of spontaneous dizygotic twinning. Analyzing data from over 700,000 individuals across Australia, New Zealand, Europe, and the United States, researchers identified four new genetic loci—GNRH1, FSHR, ZFPM1, and IPO8, which are linked to an increased likelihood of DZ twinning. These are in addition to previously known loci, FSHB and SMAD3, associated with ovarian function and reproductive hormone response. Among the new genes, GNRH1 and FSHR have known roles in female reproduction, while ZFPM1 and IPO8 are newly implicated, with IPO8 functionally validated in zebrafish as essential for female fertility.
The findings show that the tendency to have DZ twins runs in families and varies between populations, with significant genetic correlations found between twinning and traits like body size and hormone levels. Other things like race/ethnicity, height, diet, and how many children a woman has had before, may also affect the chances.
Fraternal twins may run in families. If you have a close female relative, like a sister or mother, who had fraternal twins, you may be more likely to have them too. This is because fraternal twins result from a process called hyperovulation, where the ovaries release more than one egg during the same menstrual cycle.
Hyperovulation is influenced by both genetic and non-genetic factors. Certain genes located on chromosomes 1, 11, and 15 have been linked to an increased chance of hyperovulation. Hyperovulation is the release of two or more eggs from the ovaries during the same menstrual cycle. Normally, a woman releases one egg per cycle, but in hyperovulation, multiple eggs are released, which increases the chance of having fraternal (dizygotic) twins or other multiple births.Â
If you inherit these genetic traits, especially from your mother’s side, your body may be more likely to release multiple eggs at once, raising your chances of having twins.
In addition to genetics, several other factors can raise the likelihood of fraternal twins:
Genetic testing cannot predict whether you will have twins. While some genes are associated with a higher chance of fraternal twins, there’s no commercial or medical test that can tell you definitively. However, if you are already pregnant with twins, genetic testing can help identify potential risks and guide medical care during pregnancy.
For example, genetic counselors can assess whether twins are likely to share a placenta, whether they are at risk for twin-related complications, and offer personalized guidance throughout the pregnancy.
