Exposure to chemical compounds may be the reason that some couples aren't able to achieve pregnancy, says a new study from the National Institutes of Health (NIH).

Researchers found that pregnancy is delayed in couples who have high levels of PCBs and similar environmental pollutants in their bodies by 20% compared to couples with lower exposures.

PCBs (polychlorinated biphenyls) belong to a class of chemicals called persistent organochlorine pollutants.

PCBs were widely used until 1970s in inks, paints, lubricants and electrical equipment. These chemicals have been known to increase risk of cancer. The chemical isn't being used now, however it is still released from hazardous waste sites, old transformers and improper disposal of some equipment. These chemicals are then washed away in water and stay in the environment for long periods.

"Our findings suggest that persistent organochlorine pollutants may play a role in pregnancy delay," said the study's first author, Germain Buck Louis, Ph.D., director of the Division of Epidemiology, Statistics, and Prevention Research at the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) at NIH.

PCBs were manufactured in Anniston, AL, in 1971 and have severely contaminated the environment. Studies have shown that the Anniston population is at high risk of developing diabetes.

Some chemical compounds known as persistent lipophilic organochlorine pollutants, can accumulate in fatty tissues. People can limit their exposure to these chemicals by reducing consumption of animal products and by avoiding fat in the meat.

501 couples from four counties in Michigan and 12 counties in Texas, from 2005 to 2009, were recruited for the study. All the couples were part of Longitudinal Investigation of Fertility and the Environment (LIFE) study.

Blood samples were taken and analyzed for the presence of organochlorines (PCBs) and perfluorochemicals (PFCs). Women were also asked to record their monthly menstrual cycles and results of pregnancy tests. Couples were followed till either pregnancy or after one year of their trying to become pregnant.

Researchers found that increasing concentrations of compounds in the blood reduced the odds of the couple achieving pregnancy. People exposed to PCBs had the lowest chance of getting pregnant; each standard increase in the blood concentration of the compounds resulted in lowering the probability of achieving pregnancy by 18 to 21 percent in women.

The study is published in the journal Environmental Health Perspectives.