Women with a certain kind of parasitic worm are more likely to fall pregnant and have more children, scientists have found.
A roundworm living inside women in Bolivia is now believed to cause them to mother two more children than average, the study in journal Science showed.
The study of 986 indigenous women suggested a lifetime carrying the worm, called Ascaris lumbricoides, changes the immune system to make women more fertile.
Professor Aaron Blackwell, one of the authors of the study and a researcher at the University of California Santa Barbara, told BBC News website the worm's effects were unexpectedly significant.
"We think the effects we see are probably due to these infections altering women's immune systems, such that they become more or less friendly towards a pregnancy," he said.
He added that more research would have to be carried out before considering the "intriguing possibility" that the worm could be used to increase fertility.
Another fertility scientist, Professor Allan Pacey at the University of Sheffield, said the results could lead to new drugs for women wanting to become pregnant.
"Whilst I wouldn't want to suggest that women try and become infected with roundworms as a way of increasing their fertility, further studies of the immunology of women who do have the parasite could ultimately lead to new and novel fertility enhancing drugs," he told the BBC.
Another worm, the hookworm, was found in the study to lead to fewer children across a woman's lifetime - probably because this particular parasite suppresses the immune system, rather than enhances it.
Professor Rick Maizels, a specialist in parasitic worms, told the BBC the effects of the hookworm on women's fertility were shocking.
"It's horrifying that the hookworm effects are so profound, half of women by 26 or 28 have yet to fall pregnant and that's a huge effect on life," he said.
Women's immune systems naturally change during pregnancy, scientists have previously reported.
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