Major manufacturers who use the chemicals are participating in a voluntary phase-out of them. Measuring PFC Levels Olsen and his colleagues took blood samples from 1,240 women during their first pregnancy visit, when they were about four to 14 weeks pregnant, and measured concentrations of the chemicals.

The women were part of the Danish National Birth Cohort, a study. Meanwhile, representatives of manufacturers who use the chemicals say the study shows only an association, not a cause and effect, as Olsen also emphasizes. The researchers asked the women how long it took to become pregnant; they defined infertility as a "time to pregnancy" of longer than 12 months or the need for infertility treatment to become pregnant. Olsen heads the Danish National Birth Cohort at the University of Aarhus in Denmark. The study is published online in the journal Human Reproduction. "In our study, 75% of the women had concentrations that were associated with a longer waiting time [to pregnancy]," says study researcher Jorn Olsen, MD, PhD, professor and chair of epidemiology at the University of California School of Public Health.

In the study, the higher the concentrations of these chemicals -- called perfluorinated chemicals (PFCs) -- in the women's blood samples, the more likely the women were to take more than 12 months to get pregnant.