Their findings are reported in Journal of Clinical Investigation.

Miscarriage or an ectopic pregnancy could result from smoking cannabis in the early stages of pregnancy, the scientists warned, because of the effect of THC, or tetrahydrocannabinol, the ingredient in cannabis that produces the high. It disrupts the balance of natural signaling molecules that govern implantation and development of an embryo in the womb, they explained.

Ingesting THC could alter the delicate balance required during embryonic development because it is chemically similar to cannabinoids.

One of these, anandamide, is necessary for safe passage of the fertilized egg through the fallopian tube and into the womb. In their experiments with mice, the researchers demonstrated that altering the amount of anandamide significantly affected the chance of normal implantation and survival of embryos.

The researchers continuously injected nine pregnant mice with THC. They later discovered that the embryos in all of them had been stuck in the oviduct -- the equivalent of an ectopic pregnancy in humans. In the mice that were not exposed to THC, embryos traveled to the womb normally.

There were also more abnormalities in the cells of the THC-treated mice, which suggested that they might have developed problems leading to miscarriage if their pregnancies had been allowed to proceed.

In separate research, Herbert Schuel at the State University of New York in Buffalo, reported that some new medicines, including the weight-loss drug rimonabant, marketed as Acomplia, also have the potential to disrupt amandine levels and embryo development.