The experimental vaccine contains part of a hormone manufactured by the fertilized egg early in pregnancy. It is required for the pregnancy to continue. Thirty sterilized women who received the vaccine suffered no significant side effects and developed chemical antibodies against the hormone, according to a report in the current issue of the British medical journal The Lancet.

Studies of the vaccine in baboons suggest that it works by stimulating production of antibodies that block the action of a hormone, human chorionic gonadotropin (HCG), which is released by the fertilized egg as a signal to the body that conception has occurred.

The antibodies bind to the hormone to prevent implantation, the process in which the fertilized egg embeds itself in the lining of the uterus, said Vernon C. Stevens, a professor of obstetrics and gynecology at Ohio State University's College of Medicine who developed the vaccine.