Cannabis use around the time of conception may prevent women getting pregnant, a study of mice suggests.
Smoking cannabis may affect conception, say researchers
University of Nashville researchers found an ingredient in the drug binds to receptors present in early embryos.
The ingredient prevents transport and implantation of embryos in the womb by interfering with the delicate system.
Smoking cannabis may lead to miscarriages or ectopic pregnancies, they warn in the Journal of Clinical Investigation.
The cannabinoid receptors, CB1 and CB2, are present in the brain.
When the psychoactive ingredient tetrahydrocannabinol (THC) binds to those receptors it kick-starts a chain of biological events which leads to the "high" produced by smoking cannabis.
The receptors are also present in sperm, eggs, and newly formed embryos.
During early pregnancy a naturally occurring molecule called anandamide, required for normal embryonic development, activates CB1 and CB2.
Professor Sudhansu Dey and colleagues from Vanderbilt University in Nashville, Tennessee, found that the production and breakdown of amandamide is carefully controlled and too much or too little can prevent normal embryo development, transport into the womb and implantation.
Further studies in mice showed that THC has the same effect as too much anandamine, swamping the finely tuned signalling system and causing implantation of the embryo to fail.
Professor 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."
He said all the embryos in the mice given THC failed to leave the ovary.
He added: "Our present findings have high clinical importance, since embryo retention in the fallopian tube is a significant cause of ectopic pregnancy in women, the incidence of which has markedly increased during the past decade."
"I think the effect should not be permanent but smoking marijuana means the THC level might go up and interfere with early pregnancy, even before fertilisation."
Dr Virginia Beckett, consultant obstetrician and gynaecologist at Bradford Teaching Hospitals NHS Trust and spokesperson for the Royal College of Obstetricians and Gynaecologists, said the findings made a lot of sense.
"It's really interesting. 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."
She said it would be very difficult to tease out the effect of cannabis on fertility in humans as there were many other factors, but women should certainly avoid smoking the drug if they want to get pregnant.
"Ectopic pregnancies are one in 100 and they can have a huge effect on future fertility," she said.