Tsimane women are tested annually for parasites and given medical treatment. (Courtesy Hillard Kaplan)

The Tsimane women of Bolivia are often revered as among the most fertile in the world — on average having 10 children in their lifetimes — but some are even more fertile than others.

While collecting information from nearly 1,000 women in this community over nine years, researchers from the University of California at Santa Barbara discovered that it may have to do with something pretty surprising: parasitic worm infections.

Women who were chronically infected with roundworm had as many 12 children. In contrast, women who had successive hookworm infections saw births drop to seven.

Writing in the journal Science this week, lead author Aaron Blackwell and his colleagues theorized that the effects "may relate to the balance of immune responses that the different worms induce."

Ascaris lumbricoides is the largest nematode (roundworm) (CDC)
Ascaris lumbricoides is the largest roundworm. (Centers for Disease Control and Prevention)

Infection with roundworm was associated with earlier first births and shortened inter-birth intervals, but infection with the hookworm was associated with delayed first pregnancy and extended inter-birth intervals. Blackwell told the BBC that the immune changes in a woman's body may make it "more or less friendly towards a pregnancy."

Could this be better than IVF? Fertility experts said the findings could lead to new types of treatments for women having trouble getting pregnant in the future, but that under no circumstances should women try to increase their chances of getting pregnant by actually trying to get infected with roundworms. First of all, the results are very preliminary, they only show an association rather than a causal link, and researchers don't know what mechanisms may be at play. While infections with roundworms are very common and most cases are asymptomatic, they can lead to shortness of breath, fever, anemia and in some cases fatal complications.

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