Dutch researchers have discovered why women are more likely to conceive non-identical twins as they get older. Their findings, published early online in the journal Human Reproduction, show that as women age, rising levels of follicle stimulating hormone (FSH) can push their ovaries into 'overdrive'. This increases the chances of two eggs being released in the same monthly cycle, making non-identical twin pregnancies more likely. The study authors, who are based at Vrije University Medical Centre in Amsterdam, say that about half the recent rise in multiple births in Holland is down to naturally-conceived multiple pregnancies.
The scientists studied egg follicle development in 959 spontaneous cycles of 507 women undergoing intrauterine insemination, because of unexplained or mild male infertility. They found evidence of more than one follicle developing per cycle, inferring the release of more than one egg, in 105 of the participants. Of these, only five were under the age of 30, while 45 were aged between 30-35, and 55 were aged over 35. The level of FSH - the hormone that triggers egg follicle growth - also increased with age, and was higher in women who produced multiple follicles.
The scientists say that the amount of FSH increases to counteract failing ovaries as women age, but that the hormone can 'overshoot' the threshold needed for ovulation. This increases the chances of double ovulation, say the team, making twins more likely. However, it doesn't always lead to multiple pregnancies, because of the decreasing number of 'good quality' eggs in the ovaries of older women.
Co-author Cornelius Lambalk has documented the rise in multiple births over the past decade, and says it is not entirely down to increased numbers of people having fertility treatment. 'About half of the increase has been caused by the number of spontaneous multiple pregnancies, probably due to the fact that women are delaying childbirth to a later age', he said in a press release.
Dr Gillian Lockwood, medical director of the UK's Midland Fertility Services, said the study confirmed that 'Mother Nature is doing what we deliberately strive for with ovulation induction or IVF when we use injections of FSH to produce several follicles in one cycle'. She told the BBC News website that given current concern over declining birth rates, 'it is interesting to speculate whether this natural mechanism may compensate to some extent for the 'baby gap''.