Sheffield University researchers say that a woman who has a twin brother is likely to have 25 per cent reduced fertility. A study conducted by them, published in the Proceedings of the National Academy of Sciences, blames exposure to male hormone testosterone in the womb for this.
Lead researcher Dr Virpi Lummaa tells that both testosterone and the female hormone oestrogen can cross the womb, due to which the twin brother and sister are exposed to each other's hormones. Since male and female foetuses have similar oestrogen levels, the female child is more likely to be affected, she says.
With a view to determine the effects of testosterone on female fertility, the researchers studied Finnish medical records spanning 1734 to 1888. The pre-industrial population was chosen because advanced healthcare and assisted conception treatments like IVF could have skewed fertility data on modern Western societies.
It was observed that of 754 twins, females with a twin brother were 25 per cent less likely to have children than females with a twin sister. The researchers also noted that women with a male twin were 15 per cent less likely to marry.
The findings could have resulted because, says Dr. Lummaa, females exposed to a male twin had developed masculine traits, attitudes and behaviours that affected their decision to get married. She says that exposure to high levels of testosterone in the womb increases the risk of diseases that compromise fertility, such as reproductive cancers.
"As a consequence of a male twin's influence on a female's fertility, mothers who produce opposite sex twins have fewer grandchildren and hence lower evolutionary fitness," the BBC quoted her as saying.
The researchers say that experiments on animals have shown evidence that testosterone may potentially damage female fertility.
They, however, admitted that more research was needed to look at this mechanism in humans.
"There is some evidence to support this observation. Exposure to testosterone in the womb in sheep recreates a similar syndrome to a condition called PCOS which is a known cause of infertility in humans," says Dr Laurence Shaw, a fertility expert at the London Bridge Fertility Centre and spokesman for the British Fertility Society. "Work is now needed to explore what mechanisms could explain these observations in humans," he added.