Up to a quarter of Caesarean sections could be avoided if women having difficult labours were treated differently, researchers suggest.
Acid levels may dictate if women can give birth naturally
Liverpool University experts found women whose wombs were not contracting properly had tired muscles, signified by high levels of lactic acid.
They warn treatments aimed at speeding up labour may exacerbate this problem.
The team suggest not giving treatment could allow the womb to rest and regain strength for a natural labour.
Around 21.5% of births in the UK are currently by Caesarean. Up to a quarter of those take place because labour fails to progress properly.
Researchers from Liverpool University and Liverpool Women's Hospital took blood samples from the wombs of 72 women who had undergone Caesarean sections.
A third of the operations were planned. But the rest were carried out because other assistance had failed.
The researchers found blood acidity levels were highest in women whose womb had failed to contract and for whom hormone therapy had not worked.
These women also had a higher level of lactic acid and a lower level of oxygen in their bloodstream than any of the other groups.
The researchers say if muscles are working hard but not getting as much oxygen as they need, they change their biochemical make-up so they can still perform.
This change produces lactic acid.
Women who are having difficulty in labour are currently given a synthetic version of the hormone oxytocin to speed up contractions.
But this often fails, leading to them needing a Caesarean.
The researchers say this may be because women's uterine muscles are too tired, and therefore are not capable of working any harder.
They say if women with high lactic acid levels could be identified, they would then not be given the treatment, and their bodies could be given time to recover.
'No immediate change'
Professor Susan Wray, of Liverpool University's Department of Physiology, who led the study, told BBC News Online: "Attention often seems to be focussed on the 1% of Caesareans given to women 'too posh to push', whereas a huge number are carried out on women who don't want them but end up having one.
"It may be that there is some genetic difference in these women which makes them produce more lactic acid, or something makes them produce more and they aren't able to get rid of it.
"If we knew that, we could at least advise these women to prepare themselves for the likelihood that they would be unable to have a natural birth, which would be psychologically better for them.
Obstetrician Dr Siobhan Quenby, who worked on the study, said: "With further research, the team hopes to establish what factors contribute to lactic acid build-up in the first place, enabling mothers to enjoy a natural birth without surgery."
Miss Jane Thomas, of the Royal College of Obstetricians and Gynaecologists, who was involved in drawing up recent guidelines on Caesareans, said: "This is exciting research.
"At the moment we don't have many options, and we don't have many things we can do.
"We do use oxytocin quite a lot, but there's not very good evidence that it's effective in preventing Caesareans."
But she said that, as with any potential treatment, the suggestion that women should be left to rest would need further investigation before it was adopted.
"This research offers a promising avenue to explore, but we're not going to change policy on the labour ward next week."