(Reuters) - The U.S. Food and Drug Administration changed the rules governing how prescription drugs and biologics used during pregnancy and breastfeeding are labeled.

The decision, which has been in the works since 2008, will impact more than 6 million pregnancies in the United States involving women who, on average, need to take three to five prescription drugs during gestation.

The rule published on Wednesday creates three detailed subsections on the labels of these drugs entitled "Pregnancy", "Lactation" and "Females and Males of Reproductive Potential". These subsections will replace the current system of using the product letter categories A, B, C, D and X to classify the risks of using prescription drugs during pregnancy.

"The letter category system was overly simplistic and was misinterpreted as a grading system, which gave an over-simplified view of the product risk," said Sandra Kweder, deputy director of the Office of New Drugs in the FDA's Center for Drug Evaluation and Research.

Under the new rule, the Pregnancy subsection will provide data on dosing and potential risks to the developing fetus, and require manufacturers to provide information on whether there is a registry that collects and maintains data on how pregnant women are affected when they use the product.

Information on drug labels about the existence of pregnancy registries has been previously recommended but not required until now.

The Lactation subsection will offer information about taking the drug while breastfeeding, such as the amount of drug in breast milk and the implications for breastfeeding infants.

The final subsection will include information about pregnancy testing, contraception and infertility as it relates to the drug. The changes are to come into effect by June 30 and will require newly approved drug and biologic applications to employ the new format immediately, while amended requirements for older products will be phased in gradually.

The FDA is also issuing a draft guidance for the industry to help manufacturers comply with the new labeling requirements.

(Reporting by Natalie Grover in Bengaluru; Editing by Simon Jennings)