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Two Genes In Women Increase Chances Of Conceiving Fraternal Twins

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The likelihood of having fraternal twins is in your genes, a new study has revealed.

Scientists from the Vrije Universiteit (VU) Amsterdam have postulated that two genetic variants related to processing and production of hormones for egg cell maturity increases the chance of fraternal twin conception.

How Are Fraternal Twins Made?

Fraternal twins can happen when the ovaries release two eggs that are fertilized by two different sperm.

Several factors contribute to multiple pregnancies such as race, maternal age, in-vitro fertilization, and heredity.

In the 2014 U.S. Centers for Disease Control and Prevention data, twin birth rate doubled to 33.9 in every 1,000 births from 18.9 per 1,000 births in 1980. Experts attributed this to increased use of fertility treatments.

For a long time, scientists have known that a woman's genotype has greater influence on twin pregnancies — women with familial history of fraternal twins, particularly in female relatives, have a higher chance of having twins of their own. This suggests that twin pregnancies can be due to genetic variations.

With this in mind, biological psychologist at VU Amsterdam Dorret Boomsma and her team wanted to know which genes increases the chance of conceiving twins, if there is any.

Initially, the researchers looked for individual single nucleotide polymorphisms (SNPs) that differ from one person to another.

The Genetics Of Fraternal Twins

They conducted a genome analysis of 1,980 women who conceived fraternal twins naturally and compared the data against the genes of 12,953 women with single pregnancies. The analysis revealed that two genetic variants or SNPs are much more common in those who had twins.

The team noted that DNA regions adjacent to the FSHB gene is associated with increased levels of follicle stimulating hormone (FSH), which helps egg cell maturity. They hypothesized that if more FSH is produced, there is an increased likelihood that two eggs will be released per cycle. In a regular cycle, a woman releases only one egg after it has matured.

It was also noted that SMAD3 gene affects the receptive sensitivity of the ovaries to FSH signaling. Women with SMAD3, even with average levels of FSH, still release two egg cells per cycle because of the increased sensitivity to the FSH.

Molecular geneticist at VU Amsterdam and lead study author Hamdi Mbarek said that SMAD3 effect on FSH signaling could be the reason why some women respond better to in-vitro fertilization.

The presence of the first gene increases twinning by 18 percent, the second variant by 9 percent. When both present, the chance of conceiving fraternal twins increases to 29 percent. The scientists believe that FSHB and SMAD3 are just a part of a bigger picture.

"There is a very clear suggestion and indication that more loci are contributing to the risk of having dizygotic twins as well," said Boomsma. "We've characterized the first two that have been replicated."

The study was published in The American Journal of Human Genetics on April 28.

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