Levels of a naturally occurring cannabis-like compound rise sharply in a woman's body when she is about to go into labour, research has found.
Prematurity is linked to bad health
Scientists from Leicester University believe the finding may help to identify women at risk of giving birth prematurely.
It may also help to explain why smoking cannabis is linked to a greater risk of premature labour.
Details are published in the Journal of Clinical Endocrinology and Metabolism.
Endocannabinoids are naturally occurring cannabis-like compounds, which are produced by many organs in the body.
They are recognised to play important physiological roles, but most of these are poorly understood.
The Leicester team found that levels of one particular form, anandamide, are higher before a woman ovulates than afterwards.
More significantly they found that blood levels of the compound fall steadily during pregnancy, but rise by almost fourfold when women go into labour.
The rise was very dramatic and appears to begin with the onset of uterine contractions.
Natural pain relief
Researcher Professor Justin Konje said: "These observations have very significant implications for human reproduction.
"Could the rise be an important signal for the control of labour? Is this nature's response to pain relief for labour? How are these changes affected in those smoking marijuana?
"Since marijuana is associated with preterm labour, failure of babies to grow properly in the uterus and miscarriages, could it therefore interfere with the anandamide pathway and the resultant effects on pregnancy?"
A previous study in Italy has shown that women are more likely to miscarry if they had low levels of the enzyme responsible for breaking down anandamide.
Professor Konje said a better understanding of anandamide and this associated enzyme was likely to help improve the chances of successful implantation and reduce the rate of miscarriage.
"This work is a breakthrough as it opens up several areas of research with potential implications for women with infertility, miscarriages and various complications of pregnancy.
"Most importantly, we believe that the observations could provide a new means of identifying women in true preterm labour, a complication which causes death in babies and major disability in children."
Dr Heike Rabe, a neonatologist from Brighton and Sussex University Hospitals, told the BBC News website the role of endocannabinoids in embryo implantation, pregnancy and the onset of labour had been the focus of much recent research.
She said: "There is a high concentration of the endocannaboid anandamide in the uterine tissue which also contains a high number of receptors for it.
"There is a controversial discussion in the literature whether endocrine cannaboids could play a role in miscarriage or early onset of labour.
"So far, the endocannaboids could mostly be measured in tissue samples only.
"It is therefore important that a blood test is now available so that blood levels could be measured several times during pregnancy.
"Professor Konje has outlined the questions to be answered in future research. If the research projects are successful, they could potentially contribute to the prevention of miscarriages and/or premature onset of labour in the future."