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, the number of twin births has risen 76% since 1980. It is estimated that 1 in 30 babies born in the US is a twin. (Keep in mind that this doesn’t correspond to 1 in 30 pregnancies being twins since two babies being born skews the birthrate stats.) [ref] 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.
So what does genetics have to do with twinning? It turns out that there are a couple of genetic variants that increase the probability of having fraternal twins. There seems to be few or no genetic factors 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:
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 23andMe data for rs11031006 (v4, v5):
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 23andMe data for rs17293443 (v4, v5):
Not a lot of lifehacks here :-) Enjoy your blessings if you end up with twins.
One thing to note is that rs11031006 (A/A genotype) is also associated with an increased risk of PCOS. The researchers found that the variant causes a decrease in FSH and higher luteinizing hormone (LH). [ref]
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