Parenting parasite? Intestinal worm makes women more fertile, study says

The idea of becoming infected with a parasitic worm is enough to make most people's skin crawl, but a new study says that giant roundworms have an unexpected side-effect – they can make women more fertile.

The research, led by the University of California Santa Barbara and published Friday in the journal Science, showed that women who were repeatedly infected with the giant roundworm Ascaris lumbricoides had up to two more children than those who had no parasitic infections.

The scientists studied nine years' worth of data from 986 women from indigenous Tsimane women living in the Amazon rainforest in Bolivia.

The team looked at the natural fertility rates among the women, 70 percent of whom had a parasitic infection. On average, the birth rate was nine children per woman – but those infected with the giant roundworm averaged two extra children.

The parasite, which can grow to lengths of up to 36 centimeters (14 inches), was also found to shorten the time between pregnancies and reduce the age at which women first gave birth.

The researchers hypothesized that the roundworms altered the women's immune systems, making it easier for them to become pregnant.

"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,” Professor Aaron Blackwell told the BBC, noting that women's immune systems natural change during pregnancy so their bodies don't reject the fetus.

Blackwell and his fellow researchers also suggested that the worms may aid in conception and implantation of the embryo in the womb by reducing inflammation.

Meanwhile, another parasite, the hookworm, was found to decrease fertility, leading to an average of three fewer children in a lifetime.

More than 1 billion people are infected with intestinal worms, mainly in tropical areas with poor sanitation. The giant roundworm, one of the most common, resides in the small intestine of its host and eats a portion of the person's food.