Hookworm infection was found to reduce the likelihood of pregnancy, while roundworm infection increased it.
Led by Prof. Aaron Blackwell, of the Department of Anthropology at the University of California-Santa Barbara (UCSB), the study found infection with different species of helminths - a class of intestinal parasitic worms - can either positively or negatively impact a woman's fertility.
Helminth infection is widespread in developing countries; it is transmitted through eggs present in human feces, which contaminate soil in areas with poor sanitation. The World Health Organization (WHO) estimate that around 2 billion people across the globe are infected with soil-transmitted helminths.
The researchers decided to embark on their study after one of the authors Melanie Martin - also of the Department of Anthropology at UCSB - fell pregnant while investigating parasitic worms with her husband in Bolivia.
Martin became pregnant almost immediately after her arrival in the country, which she attributed to helminth infection. This begged the question: do the parasitic worms affect fertility?
Women with roundworm infection had more children
To find out, Prof. Blackwell and his team analyzed 9 years of longitudinal data involving 986 women from Tsimane - an indigenous population of lowland Bolivia.
The researchers assessed the number of pregnancies each woman had and the incidence of infection with two of the most common helminths - giant roundworm (Ascaris lumbricoides) and hookworm (Necator americanus or Ancylostoma duodenale) - to determine whether there was a link.
Women from Tsimane have an average of nine children in their lifetime, but those infected with hookworm may have fewer.
Image credit: Michael Gurven
Around 70% of the women had helminth infection, according to the researchers, and women from Tsimane have an average of 9 children in their lifetime.
The team found that women who were infected with roundworm were more likely to become pregnant, while those infected with hookworm had a lower chance of pregnancy.
Roundworm infection reduced the length of intervals between births over the course of reproductive lifetime, the results revealed, while hookworm infection increased birth intervals.
As a result, the researchers estimated hookworm-infected women would have three fewer children than uninfected women during the course of their reproductive lifespan, while roundworm-infected women would have two additional children.
Prof. Blackwell and colleagues say it is likely that roundworm and hookworm infections affect the immune system in a way that either positively or negatively impacts the chances of conception.
Prof. Blackwell adds:
"Our findings suggest that helminth infections may have substantial effects on demographic patterns in developing populations. Further, these results may also have implications for fertility in developed populations, where many fertility problems are connected to autoimmune disorders."
Talking to BBC News, however, Prof. Blackwell says much more work needs to be done before the use of parasitic worms can be deemed a feasible treatment strategy for infertility.
Medical News Today recently reported on a study in which researchers claimed to shed light on how helminth infection increases the risk of tuberculosis.