Two genes linked to higher chance of having twins

A study has identified two gene variants that increase the likelihood of a woman having twins and could explain why twins appears to run in families. 

Using data contained in twins databases from the Netherlands, Australia and the USA, researchers at the Vrije Universiteit in Amsterdam compared DNA from over 2000 women who'd had twins with genetic information from 13,000 women who had not, and they identified a set of gene variants that might be associated with twinning.

They then sent their findings to a team in Iceland, which compared the results with their own dataset of 3500 mothers with twins and almost 300,000 controls to look for similarities.  

Two SNPs (single nucleotide polymorphisms) on the FSHB and SMAD3 genes emerged, which showed that if a mother had one copy of each the chances of having fraternal, or dizygotic, twins increased by 29 percent.

FSHB is the gene responsible for the production of follicle-stimulating hormone (FSH), which is involved in the menstrual cycle and which peaks just before ovulation. If FSH remains at high levels, it can result in more than one egg being released and an increased chance of twinning.

While the role of SMAD3 is more elusive, it seems likely that the gene affects ovarian response to FSH. If a woman's levels of FSH are normal, but her ovaries are more responsive to FSH because of a variant of SMAD3, this could also increase the likelihood of twinning. 

A genetic basis for twins has been assumed for some time from the patterns of twins that appear in families, but this research is the first to identify specific gene variants responsible.

'There's an enormous interest in twins, and in why some women have twins while others don't,' said Professor Dorret Boomsma, a biological psychologist at Vrije Universiteit and co-author of the paper. 'The question is very simple, and our research shows for the first time that we can identify genetic variants that contribute to this likelihood.'

The findings, published in the American Journal of Human Genetics, may offer some insight into the variance in implantation rates following IVF and into how multiple births, which are more common following IVF than natural conception, may be reduced in the future. Multiple pregnancies and birth present health risks for both the mother and child.

Currently, there is no way of knowing if a woman's ovaries will overproduce in response to the FSH hormone injected into the ovaries prior to IVF in order to stimulate egg production, but the researchers are now hoping to develop a genetic test to identify those women at risk of ovarian hyperstimulation, which is a dangerous side effect of IVF treatment.