Marijuana’s active ingredient may thwart pregnancy, a new study shows.

Marijuana’s key ingredient -- called tetrahydrocannabinol (THC) -- may make it hard for a fertilized egg to implant in the womb, says the study in The Journal of Clinical Investigation.

VanderbiltUniversity’s Haibin Wang, PhD, and colleagues studied mice, not people. But they write that their findings have “high clinical importance” for women.

The bottom line from Wang’s study: If you’re trying to get pregnant, abstaining from marijuana may be important for pregnancy success.

Of course, marijuana isn’t legally available in the U.S. But it’s “still one of the most widely used illicit drugs in the world, and often by pregnant women,” write Wang and colleagues.

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Tests on Mice

First, Wang’s team let female mice mate with male mice. Next, the researchers pumped THC into some of the female mice for four or five days.

For comparison, the researchers pumped other marijuana chemicals or a marijuana-free substance (placebo) into other female mice.

The mice started receiving those chemicals when their eggs had just been fertilized, but before those eggs had lodged in the womb.

The THC group was least likely to have their eggs implant in the womb, the study shows.

The scientists traced the problem down to the nitty-gritty chemical level.

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Chemical Competition

In the mice, THC docked on chemical receptors usually occupied by another compound called anandamide. That left anandamide at a loss.

Picture it this way. THC and anandamide are two types of cars prowling the mall parking lot. They both want the same type of parking spot; no other type will do.

The chemical receptor is the type of parking spot they want.

Without THC, anandamide automatically wins. But when THC is present, it swoops down and nabs the prized parking spots, leaving anandamide circling aimlessly.

The result: too much anandamide with nowhere to go.

High anandamide levels disrupt the fine chemical balance needed for a fertilized egg to move safely to the womb, note Wang and colleagues.

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Pregnancy Problems

As a result, the mice in the THC group were more likely to not have their fertilized egg implant in the womb, compared with the other mice, the study shows.

Some fertilized eggs just didn’t reach their destination. Others attached outside the womb in the ovaries or fallopian tubes, which lead from the ovaries to the uterus. That’s called an ectopic pregnancy.

In women, an ectopic pregnancy is a life-threatening condition requiring emergency treatment.

However, ectopic pregnancy has several risk factors that are totally unrelated to marijuana. Wang’s study doesn’t claim to explain all pregnancy problems in women who use marijuana.

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By Miranda Hitti, reviewed by Louise Chang, MD

SOURCES: Wang, H. The Journal of Clinical Investigation, August 2006; vol 116: pp 2122-2131. WebMD Medical Reference provided in collaboration with The Cleveland Clinic: “Ectopic Pregnancy.” News release, The Journal of Clinical Investigation.