Infection with parasitic worms may boost or reduce a woman’s fertility, researchers reported in the journal Science.

For nine years, scientists collected data from Tsimane women in Bolivia, where the average birth rate is nine children per woman. Women who were repeatedly infected with hookworm were likely to have up to three fewer children than uninfected women, the researchers found.

In contrast, women infected with a species of roundworm were found to have up to two more children than those without infections. Both parasites seem to invoke immune changes in the human body; the researchers believe that roundworms, for instance, may cause an increase in certain immune cells that are beneficial during pregnancy.