If a woman's female relatives have fraternal twins, she is more likely to give birth to twins herself, but the genes behind this phenomenon have remained a mystery.
Researchers have now shown genetic links between having twins and female production of, and response to, follicle-stimulating hormone, which may help predict how some women respond to infertility treatments.

An international team of researchers including scientists from Vrije Universiteit (VU) in Amsterdam aggregated genetic data from twin databases in the Netherlands, Australia and the US. The sample totalled 1,980 mothers of fraternal twins conceived without fertility treatments and 12,953 controls.

Researchers were looking for genetic variants, shared by mothers with twins, that showed a different frequency from those in the control groups.
Once researchers had identified a handful of candidates, they sent the results to collaborators in Iceland, who crunched the numbers on their own set of 3,597 mothers with twins and 297,348 controls.

"They looked for whether these variants were also significant in their cohort," said Hamdi Mbarek from VU.

Two of the gene variants were replicated in the Icelandic cohort, appearing more often in the mothers with fraternal twins conceived without fertility treatments, researchers said.

One of the variants, located near a gene called FSHB, is associated with higher levels of follicle-stimulating hormone (FSH), they said.
This hormone triggers follicle growth in a woman's ovaries, eventually leading to an egg being released – and with higher FSH levels, multiple eggs may be released at the same time, leading to twins if two get fertilised, researchers said.

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