The rate of twin births has skyrocketed in the recent past, rising by 76 percent in the United States between 1980 and 2011. A large study by researchers from eight different countries that looked at 1980 mothers of fraternal twins and found that two genetic factors influence the likelihood of a twin birth.
Asia and Latin America have the lowest rate of twin births, whereas Central Africa and the United States have rather high rates, with up to thirty twin births per thousand live births. And the more twins are born, the more parents are asking themselves why they in particular had twins. Fertility treatments have resulted in more twin births, making up 36 percent of twin births in the United States alone, yet most twins are conceived naturally and spontaneously.
Researchers have long since asked themselves the same question and they might have found another puzzle piece in unraveling the mystery. In 1987 the Dutch geneticist Dorret Boomsma began the Netherlands Twin Register. More than 75,000 twins and triplets, as well as other multiple births, have been registered there since its inception.
Contrary to identical twins, which developed from the same egg and are thus genetically identical, fraternal twins are dizygotic – they developed from two different eggs, fertilized by two different sperm cells. This, in essence, just makes the babies regular siblings that happened to be born at the same time and from the same pregnancy. Why some mothers are more likely to have fraternal twins was now studied again by a team of researchers in the Netherlands.
Researchers know that in vitro fertilization, which is becoming more and more common, has a higher chance of leading to twin births, as well as the fact that older women are more likely to release more than one egg, resulting in fraternal twins, are more frequently becoming mothers. That women who have a history of twin births in their family also have increased odds to have fraternal twins has also been known for quite some time.
Other factors that seem to be common in mothers of fraternal twins is that on average they are larger and heavier, often even overweight, and many of them smoked before the pregnancy. As nutrition and periods of starvation had little to no effect on twin births suggested to researchers that genetics must play a bigger role in a woman’s chance to become a mother of twins. As risks for mother and children are much higher with twins, and result in higher medical costs, researchers are looking to find the reason for its ocurrence.
Molecular geneticist at Vrije Universiteit Hamdi Mbarek assembled a team that looked at data from nearly 2000 fraternal twin births and compared it to identical twin births and regular births. They used the aforementioned Dutch twin registry, as well as registries from Australia and a US registry from Minnesota. They examined two single nucleotide polymorphisms (SNPs), individual DNA bases in genes, that are more frequent in mothers of fraternal twins.
One of the SPNs can be found around a gene called FSHB, responsible for producing follicle-stimulating hormones. Since higher levels of these hormones can lead to the release of multiple eggs, the fact that this hormone is more common in mothers of fraternal twins did not come as a surprise.
The other one, and researchers did not expect that, was located in a gene called SMAD3. This gene, as shown by studies in mice, influences the way the ovaries react to follicle-stimulating hormones. Further studies are now needed to determine, if the presence of SMAD3 has something to do with women’s ability to get pregnant through in vitro fertilisation.
The results were published in the American Journal of Human Genetics and showed that if a woman had one copy of each of the two SNPs that were studied she is 29 percent more likely to give birth to fraternal twins. As of now, no study has been able to show a genetical link for having identical twins.