THC, the active ingredient in marijuana, may prevent pregnancy by making it harder for a fertilized egg to implant in the mother's body, WebMD reported Aug. 1.

In the study, researchers let female mice mate with males, then injected them with either THC, other marijuana chemicals, or a placebo. They found that the THC group was less likely to have their eggs implanted in the womb, probably because THC interferes with a compound called anandamide, which is involved in embryo implantation. Some eggs didn't implant at all, while others implanted ectopically -- a dangerous situation where the fetus starts to grow in the ovaries or fallopian tubes.

Researcher Haibin Wang, Ph.D., of Vanderbilt University said that the findings should encourage women to abstain from marijuana if they're trying to get pregnant.

The research appeared in the Aug. 1, 2006 issue of the Journal of Clinical Investigation.