Women who smoke cannabis in the early stages of pregnancy could risk miscarriage or an ectopic pregnancy, according to a study. Experiments on mice showed that embryos in females given the drug during pregnancy were more likely to develop abnormally and not make it to the uterus. THC, the chemical in cannabis that gives the high, alters the balance of natural signalling molecules in the reproductive systems that govern development and implantation of the embryo in the womb.

"This raises concern," said Sudhansu Dey at the Vanderbilt University Medical Centre in Nashville. If the same was true in humans then smoking cannabis might mean "that pre-implantation embryo development will be abnormal and that the embryo might get trapped in the oviduct instead of going to the uterus", he said.

THC (or tetrahydrocannabinol) affects the system because it is chemically similar to signalling molecules in the body called cannabinoids. One of these, anandamide, controls development of the embryo by binding to receptors on it. The level of anandamide in the reproductive system is crucial for normal development.

But if THC swamps the system, for example in a cannabis smoker, it disrupts this fine control.

To show this effect the team set up pumps which continuously injected nine pregnant mice with THC. The researchers later killed the mice and found that the embryos in all of them had been held up in the oviduct, the tube linking the ovary to the uterus. This is the equivalent of an ectopic pregnancy in humans. In mice not exposed to THC the embryos proceeded to the womb normally. The embryos in THC-treated mice also had more cell abnormalities, suggesting developmental problems that would lead to miscarriage. The results are reported in the Journal of Clinical Investigation.

The results echo research on tobacco smokers, showing that they are more prone to ectopic pregnancies. Experiments on hamsters have shown that nicotine and tobacco smoke prevent normal transport of embryos to the uterus.

Men do not escape either. Previous research has found that cannabis smokers produce less semen, containing fewer sperm. THC binding to cannabinoid receptors on sperm can prevent binding with eggs. Professor Dey also points out that his findings suggest problems for obesity drugs which affect the cannabinoid system. Because these can bind to the same chemical receptors in the body as its own cannabinoids, he worries that they may have the unintended consequence of harming a woman's fertility.