Health officials in the U.S. will study the effects of prescription drugs taken during pregnancy.

The U.S. Food and Drug Administration announced the study, called the Medication Exposure in Pregnancy Risk Evaluation Program, on Wednesday.

About two-thirds of women who deliver take at least one prescription medication, according to an article in the American Journal of Obstetrics and Gynecology.

But few clinical trials test the effects on women and fetuses.

"Results of these studies will provide valuable information for patients and physicians when making decisions about medication during pregnancy," Gerald Dal Pan, a director at the FDA's Center for Drug Evaluation and Research, said in a statement.

Investigators will use data from 11 research sites in the U.S. affiliated with large health plans. The sites have health-care information for about one million births from 2001 to 2007.

The FDA gave no timeline for completing the study.