The egg normally travels down the fallopian tube from the ovary to the womb

Reactions to cannabinoid chemicals were important for the embryo to move along the fallopian tube towards the womb, animal experiments showed.

The Vanderbilt University scientists told Nature Medicine their findings could be important for understanding why ectopic pregnancies can occur.

This is when the embryo starts to grow outside the womb.

Ectopic pregnancy

The researchers recommended more research to find out whether cannabis use might increase the risk of ectopic pregnancies.

In normal pregnancy, eggs make their way from the ovaries to the womb through the fallopian tubes, where they may be fertilised by a sperm.

The fertilised egg continues on to the womb, where it implants itself to the wall and continues to grow.

In ectopic pregnancy, the fertilised egg takes an alternative route and can implant itself outside of the womb.

Most often it implants in the fallopian tubes themselves, but it can also latch on to the ovary, the abdomen, or the cervix.

According to Dr Sudhansu Dey and colleagues, marijuana is one of the most popular illicit drugs used by pregnant women, raising concern about its affects during pregnancy.

Our results raise caution for women of reproductive ages regarding chronic marijuana consumption for recreation or pain alleviation

The study authors

Researchers had already found receptors in mouse embryos that respond to cannabis-like chemicals.

The same receptors have also been found in the wombs of humans.

Dr Dey's team decided to investigate the role of these receptors by blocking their action.

When they were blocked the normal journey of mouse embryos along the fallopian tube towards the womb was disrupted.

Cannabis

The researchers said their findings had major implications for ectopic pregnancy in women because the fallopian tubes the site where most occur.

"It remains to be seen whether the incidence of ectopic pregnancy is associated with...chronic use of cannabinoids," they said.

It's such a neglected area of research, despite one pregnancy in every 80 being ectopic

Professor Bill Ledger, consultant obstetrician and gynaecologist at Sheffield University

But they added: "Collectively, our results raise caution for women of reproductive ages regarding chronic marijuana consumption for recreation or pain alleviation."

They said tobacco smoking was already known to increase the risk of ectopic pregnancy and had been shown to impair the journey of rat embryos along the reproductive tract.

Professor Bill Ledger, consultant obstetrician and gynaecologist at Sheffield University, said: "It's very interesting and it would merit more human research to see if cannabinoid receptors are present in the human fallopian tube.

"But to say that women who smoke cannabis socially are at high risk of ectopic pregnancy would be a step too far at this stage.

"The major causes of ectopic pregnancies are sexually transmitted infections and other things that damage the fallopian tubes like endometriosis and previous surgery.

"We know the risk factors and we are getting better at detecting ectopic pregnancies and treating them.

"But we need much more research like this looking at exactly why they occur.

"It's such a neglected area of research, despite one pregnancy in every 80 being ectopic," he said.

Sue Jacob from the Royal College of Midwives said: "We need to look at the bigger picture.

"The rise in sexually transmitted infections, the rise in alcohol binging and the rise in drug taking all have an impact on sexual health.

"Ectopic pregnancies need to be researched further in light of these behaviours," she said.