Twins tend to ‘run in families’, which is a colloquial way of saying that there is a hereditary component that increases the likelihood of having twins.
According to the CDC, twin births have risen 76% since 1980. Estimates show that 1 in 30 babies born in the US are twins. (Keep in mind this doesn’t correspond to 1 in 30 pregnancies being twins since two babies being born skews the birthrate stats.)[ref] As a matter of fact, this increase is due to better prenatal care for pregnant moms. In-vitro fertilization (IVF) is also responsible for a portion of the increase.
So what does genetics have to do with twinning? There are a couple of genetic variants that increase the probability of having fraternal twins. It seems few or no genetic factors are involved in having identical twins.[ref]
Quick terminology primer: Identical twins come from one fertilized egg that splits into two after fertilization. These are called monozygotic (one zygote). Fraternal twins come from two different egg cells being released and fertilized. These are called dizygotic twins.
Fraternal Twin Genes:
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The follicle-stimulating hormone beta-subunit (FSHB) gene codes for a protein that is part of the process to stimulate the release of an egg cell during ovulation.
Check your genetic data for rs11031006 (23andMe v4, v5):
- A/A: lower odds of having fraternal twins[ref]
- A/G: lower odds of having fraternal twins
- G/G: typical odds of having twins
Members: Your genotype for rs11031006 is —.
SMAD3 is involved in initiating growth factors that regulate cell proliferation and differentiation. This variant may influence how the ovary responds to follicle-stimulating hormone.
Check your genetic data for rs17293443 (23andMe v4, v5):
- C/C: 19% increase in the possibility of having fraternal twins[ref][ref]
- C/T: 9% increase in the possibility of fraternal twins
- T/T: typical
Members: Your genotype for rs17293443 is —.
Not a lot of lifehacks here :-) Enjoy your blessings if you end up with twins.
One thing to note, rs11031006 (A/A genotype) shows an association with an increased risk of PCOS. The researchers found that the variant causes a decrease in FSH and higher luteinizing hormone (LH).[ref]
Recap of your genes:
|Gene||RS ID||Risk Allele||Your Genotype||Notes About Risk Allele|
|FSHB||rs11031006||A||--||Lower odds of having twins|
|SMAD3||rs17293443||C||--||Increased possibility of fraternal twins|
Related Genes and Topics:
MTHFR: How to check your data
It is easy to check your genetic results on 23andMe or AncestryDNA for the two main MTHFR variants known as C677T and A1298C.
Dads matter: MTHFR variants in fathers affect miscarriage risk
Quite a few studies are showing that women carrying certain MTHFR variant combinations are at a somewhat higher risk for miscarriage, but I recently ran across a study that added a new twist to the topic. It turns out that the father’s MTHFR variants can also play a role in recurrent miscarriages.
Genetic links to infertility for women
Your genes may be playing a role in your infertility — and knowing which genetic variants you carry may help you figure out solutions to try.
Genetics, inflammatory cytokines, and recurrent miscarriage
Learn about how genetic variants in certain inflammatory cytokines can increase or decrease the risk of recurrent miscarriage.
Day, Felix R., et al. “Causal Mechanisms and Balancing Selection Inferred from Genetic Associations with Polycystic Ovary Syndrome.” Nature Communications, vol. 6, no. 1, Sept. 2015, p. 8464. www.nature.com, https://doi.org/10.1038/ncomms9464.
Is the Probability of Having Twins Determined by Genetics?: MedlinePlus Genetics. https://medlineplus.gov/genetics/understanding/traits/twins/. Accessed 17 May 2022.
Mbarek, Hamdi, Stacy Steinberg, et al. “Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility.” American Journal of Human Genetics, vol. 98, no. 5, May 2016, pp. 898–908. PubMed Central, https://doi.org/10.1016/j.ajhg.2016.03.008.
Mbarek, Hamdi, Conor V. Dolan, et al. “Two SNPs Associated With Spontaneous Dizygotic Twinning: Effect Sizes and How We Communicate Them.” Twin Research and Human Genetics: The Official Journal of the International Society for Twin Studies, vol. 19, no. 5, Oct. 2016, pp. 418–21. PubMed, https://doi.org/10.1017/thg.2016.53.
Debbie Moon is the founder of Genetic Lifehacks. Fascinated by the connections between genes, diet, and health, her goal is to help you understand how to apply genetics to your diet and lifestyle decisions. Debbie has a BS in engineering and also an MSc in biological sciences from Clemson University. Debbie combines an engineering mindset with a biological systems approach to help you understand how genetic differences impact your optimal health.