Australian scientists appear to have solved one of the great mysteries of human biology - exactly what triggers labour after about 40 weeks of pregnancy.
They cannot explain how the timing works or predict a child's birthday with greater accuracy.
But they know that, when it's time, the body produces a protein, which releases a safety switch that allows the uterus to contract in a way that any stretched muscle should.
This could lead to new drugs to induce labour in overweight or obese women, as well as a way to prevent pre-term births, says Professor Shaun Brennecke from the University of Melbourne and Royal Women's Hospital.
"For decades, we have been looking at what might start human labour. This is the first clear-cut identification of that.
"Rather than all the hormonal changes which have been talked about in the past, this is the actual switch that starts labour," said Prof Brennecke, who worked with colleagues at Monash University and the Hunter Medical Research Institute in Newcastle on the project.
The study, which is published in the journal Nature Communications, is aimed at finding alternatives to the dangerous caesarean sections that are common among overweight and obese mothers.
Overweight mothers do not produce enough of the protein molecule to start the chain reaction, said study leader Professor Helena Parkington of Monash University.
"These women also respond poorly to our current methods of induction."
The study gives two potential options for improving the labour of obese women, says Prof Brennecke.
There could be an existing drug that can switch off the safety mechanism, or there might be one that increases the amount of protein produced.