Are you genetically more likely to have twins?

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.

Twin Births:

According to the CDC, the number of twin births has risen 76% since 1980. Estimates show that 1 in 30 babies born in the US is a twin. (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, a lot of this increase is due to better prenatal care for pregnant moms. In-vitro fertilization (IVF) is also responsible for a portion of the increase.

CDC chart of twin births in the US.

So what does genetics have to do with twinning? It turns out 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:

FSHB gene: 
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

SMAD3 gene:
SMAD3 is involved in the initiation of growth factors that regulate cell proliferation and differentiation. This variant may influence how the ovary is responding 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



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]

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
There are quite a few studies 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.