The chemicals Fei and colleagues looked at belong to a group called perfluorinated chemicals, or PFCs, which appear in a variety of common products, from upholstery to pesticides. In particular, the researchers focused on perfluorooctane sulfonate and perfluorooctanoate, which are respectively called PFOS and PFOA.

Studies have linked PFOS and PFOA to toxic effects in the livers, immune systems, and reproductive systems of animals. In people, Fei and colleagues previously found that women with many children had lower blood levels of PFOS and PFOA than did women with fewer children. In turn, the scientists wondered if these chemicals might affect fertility. Eight percent of women in the United States have visited their doctors for infertility-related reasons, according to the Centers for Disease Control and Prevention.

To investigate, the team collected blood and surveyed more than 1,200 newly pregnant women who are taking part in the Danish National Birth Cohort, a long-term health study. All of the women had become pregnant on purpose. About 30 percent of women tried for more than six months before conceiving, results showed. Half of those tried for more than a year. There was an equally big range in chemical levels in the women's blood — with more than 40 times more PFOA in some women than others and more than 16 times more PFOS from the lowest to highest concentrations.

For analysis, the researchers divided the women into groups of high and low chemical levels. Their calculations showed that women with the most PFOS in their blood were up to 134 percent more likely to have needed six months or more to get pregnant. Women with the most PFOA were up to 154 percent more likely to have trouble conceiving.