The researchers studied CB1, a "cannabinoid" receptor that binds the main active chemical for marijuana, delta-9-tetrahydrocannabinol (THC).
In pregnant mice that lacked the gene for the receptor, or in which the receptor was blocked, the embryo failed to go through the oviduct the tube leading from the ovaries to the uterus. The same thing happened in normal mice when the receptor was over-stimulated.
The study, published in the current issue of the journal Nature Medicine, describes for the first time how the CB1 receptor in the mouse regulates muscle contraction to move the embryo down the oviduct.
It is not known whether drugs that block or, in the case of marijuana, over-stimulate the CB1 receptor can cause ectopic pregnancy in humans. However, "our results raise caution for women of reproductive ages regarding the chronic use of marijuana for recreation or pain alleviation," the researchers concluded.
The report's senior author, Sudhansu K. Dey, Ph.D., said he also was concerned about the potential impact of an anti-obesity drug, now in clinical trials, that suppresses appetite by blocking the CB1 receptor. Such a drug, if approved, would likely be taken by young women of reproductive age.
"What will happen if they consume anti-CB1 drugs?" asked Dey, Dorothy Overall Wells Professor of Pediatrics and professor of Cell & Developmental Biology and Pharmacology at Vanderbilt University Medical Center.
According to the U.S. Centers for Disease Control and Prevention, about 100,000 ectopic pregnancies occur in the United States each year (out of more than 6 million total pregnancies) and account for about 9 percent of all pregnancy-related deaths in the country. Risk factors include pelvic inflammatory disease, which can scar the fallopian tubes, and smoking.