Scientists have long known that certain factors – such as becoming pregnant at an older age or having a family history of twins – can increase women’s chances of giving birth to fraternal twins, but the underlying reasons have remained a mystery.

Hamdi Mbarek, molecular geneticist from Vrije Universiteit, set out to find answers by leading a two-part study, published in the American Journal of Human Genetics.

The team, which consisted of researchers from eight countries, analyzed data from 2,000 mothers of fraternal twins who were conceived without the use of fertility treatments, and compared their DNA to a control group of nearly 13,000 women who either didn’t have twins, or who had identical twins.

The data was collected from databases in the Netherlands, Australia and the U.S.

The researchers specifically looked for individual DNA bases known as single nucleotide polymorphisms (SNPs), which vary among individuals. They marked specific SNPs that appeared more often in mothers of fraternal twins, but not in the control group.

After finding potential SNPs that may offer the insight they were looking for, Mbarek and colleagues ran another analysis in a separate database from Iceland, which included 3,597 mothers of fraternal twins and nearly 300,000 control group mothers. The additional analysis further narrowed down their results to just two SNPs.

The first variant, the SNP called FHSB is already known to be involved in the production of follicle-stimulating hormone FSH, which encourages ovaries to release eggs. However, FSH levels fluctuate as the eggs in ovaries mature, which can result in the release of multiple eggs –the first step that can lead to fraternal twin birth.

The second SNP, called SMAD3, plays a role in cell signaling. Other studies have shown that SMAD3 can change how ovaries respond to FSH in mice, leading the researchers to believe that the gene may be a clue to why women respond differently to in vitro fertilization methods.

The results showed that having one copy of each SNP increased a mother’s chance of having fraternal twins by 29 percent.

Unlike identical twins, the chances of fraternal twins vary by region and change with time. In the U.S., the odds of having twins has risen 76 percent from 1980 to 2009. These statistics are partially due to fertility treatments.

The researchers note that there may be more genes at play that weren’t identified through the study.



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