Roundworms could hold the key to improving a woman's fertility, research suggests.
The parasitic worm, which is found in the human gut, might be responsible for altering the immune system in women - making them more likely to become pregnant.
A nine-year study of 986 women in Bolivia found that those who had been infested by ascaris lumbricoides (a type of roundworm) were likely to have more children and get pregnant more quickly than uninfected women.
Researchers believe their findings could be used in developing fertility-enhancing drugs.
The study of Tsimane women in Bolivia examined two types of worm found in their bodies - hookworm and roundworm.
Lead author Aaron Blackwell, an assistant professor in University of California Santa Barbara's Department of Anthropology, said: "We found that different species of helminths - a family of parasitic intestinal worms - could have either positive or negative effects on the timing of a Tsimane woman's next pregnancy.
"Hookworm infection tended to increase the length of the intervals between births and that was consistent across all ages. But younger women infected with roundworm had shorter birth intervals."
Researchers said women infected by hookworm have, on average, three fewer children than uninfected women. Meanwhile, women infected with roundworm are likely to have two more children than uninfected women.
"These opposing effects are likely due to helminth (worm) infection affecting the immune system, which in turn affects the likelihood of conception," said Blackwell.
He believes the findings, which were published in the journal Science, could have "implications for fertility in developed populations, where many fertility problems are connected to autoimmune disorders".
But Dr Helen Webberley, the dedicated GP for Oxford Online Pharmacy, urges women to interpret this kind of research carefully.
"This study of 986 women who have carried the parasite, does show they have had two more children, but unless this is what the study set out to ask then it is an incidental finding.
"There have been many incidental findings in medical research that have turned out to be a complete coincidence."
She continues: "In order for the study to provide meaningful results it would need to follow 986 women who have the infection and compare them with 986 who have not, specifically asking the question 'do they have more children?' or 'are they more fertile?'. The results would then give us a formal answer.
"In the meantime, for improved chances of conception, we need to rely on modern medicines such as Clomid which are evidence-based."