Crucially, the discovery, published today in the journal Nature Communications, shows that women who are overweight have a faulty switch. The finding may explain why overweight women have a higher likelihood of irregular contractions and are more likely to require a caesarean section than other women.
It's well known that strong rhythmic contractions of the uterus are needed to allow the baby's head to dilate the cervix. However little was known about what controls these contractions until now.
The groundbreaking research from Monash University, the Royal Women's Hospital and the Hunter Medical Research Institute, show that a potassium ion channel called hERG in the uterus is responsible.
Acting as a powerful electrical brake, hERG works during pregnancy to suppress contractions and prevent premature labour. However, at the onset of labour a protein acts as a switch to turn hERG off, removing the brake and ensuring that labour can take place.
The team, led by Professor Helena Parkington from the School of Biomedical Sciences at Monash University, found that in overweight women the switch doesn't work, failing to turn hERG off.
"We've known for years that women who are overweight are much more likely to experience complications during pregnancy and labour - but we didn't know why," Professor Parkington said.
"Pinpointing the mechanism is a major breakthrough, not only does it ensure a smooth pregnancy, but knowing when contractions kick in at more or less the right time, is crucial to our understanding of the labour process."
Figures from the Royal Women's Hospital show that over 50 per cent of Australian women are overweight or obese; including thirty five per cent of women aged 25-35. As a result, clinicians are seeing more pregnant women who are overweight. These women are far more likely to experience complications during labour and delivery and require medical intervention.
By analysing uterine muscle biopsies from 70 pregnant women, the team found striking differences between overweight women and those of a healthy weight.
Testing the electrical signals in small amounts of uterine tissue taken from women who had an elective caesarean before labour started and women who needed an emergency caesarean during their labour, proved that hERG was dysregulated in overweight women.
Professor Parkington said this is the first time that the brake and the protein controlling it have been identified in uterine muscle. The insight could help to reduce complications during pregnancy and labour in overweight women.
"Now we know what is responsible for malfunctioning labour in overweight women, we can look to develop treatments to correct the 'faulty' switch and ensure both safer pregnancies and labour for women and their babies," she said.
The next phase of the research will look at developing a safe, effective and specific treatment, for example a new drug that turns off the switch to allow labour to start and progress.