Components of cannabis have varied effects on the body

US scientists found that anandamide, at low concentrations, appeared to play a vital role in the implantation of the embryo into the womb lining.

However, writing the Proceedings of the National Academy of Sciences, they said that even slightly higher doses might interfere with that process.

The researchers say that the embryo is a "target" for cannabis chemicals.

Anandamide is a substance which occurs naturally in the body, as well as being one of the cocktail of chemicals released when cannabis is burned.

The strong suspicion that it naturally plays a role in fertility is boosted by the fact that the body produces more just before an embryo is meant to implant in the womb wall, and less at the time of implantation.

However, there is some evidence linking excess levels of anandamide to "spontaneous abortion" - miscarriage.

Mouse tests

The researchers from Vanderbilt University Medical Centre in Tennessee wanted to find out whether how this chemical, or cannabinoid, might be having an effect on either the womb environment or the embryo itself.

Using mouse embryos, they found a chemical pathway in the embryo triggered by low levels of anandamide.

This turned on a mechanism which allowed the embryo to implant.

However, if the concentration of the chemical in contact with the embryo was increased only four-fold, the reverse happened, and embryo implantation was hindered.

'Significant'

The researchers, led by Dr Sudhansu Dey, wrote in the journal: "These results are relevant to humans because spontaneous pregnancy losses occur in women with elevated anandamide levels.

We know that cannabinoids affect sperm - although there's not much objective evidence that it has an effect on female fertility Professor John Oxford, Imperial College London

"Regulated endocannabinoid signalling is at least one of the pathways that determine the fate of embryo implantation

"This study places the embryo as a target for natural and endocannabinoids (e.g. those from cannabis smoke), and raises the significance of cannabinoid signalling in female fertility."

John Oxford, professor of virology at St Bartholomew's and the Royal London School of Medicine, told BBC News Online that the findings were "very plausible".

He said: "We know that cannabinoids affect sperm - although there's not much objective evidence that it has an effect on female fertility.

"It's difficult to carry out a proper controlled trial to find out what is happening."

Dr Richard Henshaw, medical director of Centres for Assisted Reproduction, weclomed the study.

"When infertile couples seek help, we as a routine advise that all recreational drugs, including cigarettes and alcohol, be ceased.

"There is clear evidence that excess use of recreational drugs impairs maternal and fetal health during pregnancy.