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. This quick article explains the genetic variants linked to the likelihood of having fraternal twins.
Twin Births:
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]
Terminology:
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 Twins Genotype Report:
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FSHB gene:
The follicle-stimulating hormone beta-subunit (FSHB) gene codes for a protein that is part of the process of stimulating 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 gene:
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 (FSH).
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References:
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.