New research has suggested a link between cannabis smoking and increased risk of miscarriage or ectopic pregnancy. Cannabis smoking around the time of conception may disrupt a delicate balance of enzymes which allow the early embryo to progress along the fallopian tubes into the uterus and then implant.
A team at the Vanderbilt University Medical Centre in Tennessee, US, conducted experiments on mice by continuously injecting pregnant animals with tetrahydrocannabinol (THC), the chemical in marijuana that gives users a high. The results showed that in the earliest phase of pregnancy, embryos failed to travel along the fallopian tubes, implantation could fail and embryos were more likely to develop abnormally. THC is chemically similar to naturally occurring signalling molecules called cannabinoids, one of these - anandamide - is important in directing the early embryo. Anandamide normally binds to the cannabinoid receptors CB1 and CB2, but may be displaced by THC if it is present, levels of anandamide in the oviduct are then elevated.
Cannabinoid receptors are present in the brain, when THC binds to them a high is produced, however cannabinoid receptors are also found on sperm, eggs and embryos. Production and breakdown of anandamide is carefully controlled in early pregnancy to ensure smooth progression of the embryo along the oviduct and into the uterus to implant. Increased THC, caused by smoking cannabis, could flood the finely balanced system which may then fail. Lead researcher Professor Sudhansu Dey said, 'Our observation of defects of pre-implantation events and pregnancy failure in mice exposed to excessive THC raises concern that the adverse effects of maternal use of marijuana could be seeded very early in pregnancy'. In their experiment all of the embryos of the mice given THC failed to leave the oviduct, in mice not exposed to THC the embryos proceeded to the uterus normally. The research is published in the Journal of Clinical Investigation.
In an accompanying commentary Professor Herbert Schuel of State University in New York, an expert in reproductive biology, also notes that many appetite suppressing drugs (many of which are still in development) also work by modifying anandamide signalling. One such drug rimonabant, sold as Acomplia, is already licensed in the UK. A spokeswoman for rimonabant's manufacturer said the company did not recommend the use of the drug during pregnancy and advised patients who are planning to become pregnant to seek immediate medical advice.
Previous research has also shown that men who smoke cannabis produce fewer sperm and that THC binding to receptors on sperm can suppress their ability to successfully bind to eggs. Dr Virginia Beckett, spokesperson for the Royal College of Obstetricians and Gynaecologists, described the research as 'very interesting'. She said, 'We're really trying to focus on preconception care and we try and encourage women to get healthy before they consider conceiving. Cannabis use certainly has a huge effect on infertility in men. We're used to counselling men and it's useful to be able to use this research to advise women as well'.