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A large new study of women who have fraternal twins could help spur fertility treatments for those who can’t have any kids at all.

The research, published Thursday in the American Journal of Human Genetics, suggests that the tiniest quirk in the genome near two specific genes could make it more likely for a woman’s ovaries to produce two eggs at the same time. Those eggs, both fertilized, will become siblings born on the same day.

While the research is preliminary, it could change the course of fertility treatments.


“Ever since the first publications on twins, people have noted that twinning runs in families and have been wondering: does it have a genetic reason, or is it something in the environment? Is it the food people eat? Or exposure to sunshine?” said Dorret Boomsma, a biological psychologist at Vrije Universiteit Amsterdam, and the lead author on the study.

To figure it out, Boomsma and her colleagues looked at the genetic sequences of nearly 2,000 mothers who had had fraternal twins and around 13,000 people who didn’t have twins. They found three tiny genetic variations that were more common among the mothers of twins. By comparing it to a huge group of mothers in Iceland, the researchers narrowed the candidates down to two tiny changes in the genetic sequence near two genes.


The first, called FSHB, contributes to the production of a chemical called follicle-stimulating hormone, which prods the ovary into producing eggs. The second, called SMAD3, is involved in making the ovary receptive to this chemical, which is produced at the base of the brain.

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The findings could explain why even those women who don’t produce much follicle-stimulating hormone may still be at a higher risk for having fraternal twins — although the research doesn’t definitively map out these mechanisms.

“It’s an association,” said Dr. Nancy Rose, a geneticist and obstetrician-gynecologist at the University of Utah, who was not involved in the study. “It’s not necessarily cause and effect.”

But Rose noted that the genetic discovery could change the way doctors give fertility treatments to women who have these particular DNA variants. After all, if a woman is already genetically predisposed to give birth to twins, a clinician wouldn’t want to make that even more likely by giving her a high dose of fertility-enhancing drugs.

This research could also open doors to the creation of new fertility treatments. By narrowing down which genes are involved in these mothers’ increased fertility, researchers have new mechanisms to investigate.

As Dr. Chanika Phornphutkul, a geneticist and pediatric endocrinologist at Rhode Island Hospital, put it, “We have 20,000 genes. If we don’t find an association with a certain gene, then you don’t know where to start.”

Correction: An earlier version of this story misstated how much time it takes for twins to be born.