About 60,000 babies are born prematurely in the UK each year, which leaves them vulnerable to lifelong health problems and is a leading cause of death in new-borns.
Reasons for premature birth can be unclear, making it hard for doctors to predict and prevent.
But the charity Tommy’s is investigating clues in the mother’s vaginal microbiome (all the tiny organisms living inside, or on, the human body that can affect health, like bacteria and viruses) and also the metabolome, which is the range of small molecules naturally created by the body, like sugars and acids.
Study author Andrew Shennan is professor of obstetrics at King’s College London and runs the Tommy’s Preterm Surveillance Clinic at St Thomas’ Hospital.
He said: “Premature birth is very hard to predict, so doctors have to err on the side of caution and mothers deemed to be at risk often don’t actually have their babies early, putting undue strain on everyone involved.
“My team has developed preterm birth prediction tools that are very accurate later in pregnancy, like foetal fibronectin tests – but at that stage, you can only manage the risks, not stop it from happening.
“The sooner we can find out who’s at risk, the more we can do to keep mothers and babies safe.”
Previous research suggests infection and inflammation can shorten and weaken the cervix.
The baby is at risk of infection and premature birth if this happens earlier in the pregnancy.
In the new study, Tommy’s scientists have uncovered specific bacteria and chemicals in pregnant women’s cervicovaginal fluid that can act as warning signs.
This means infection or inflammation can be found and treated early to protect mothers and babies, the researchers suggest.
The researchers at King’s College London looked at data from four UK hospitals on 346 mothers, 60 of whom gave birth prematurely – before 37 weeks pregnant.
They analysed cervicovaginal samples taken at 10-15 weeks pregnant, and again at 16-23 weeks, then grouped women by their typical communities of bacteria and biochemicals.
This was checked against cervical length measurements (the current standard NHS assessment for premature birth risk) and followed up to see who gave birth early.
For the first time, a specific bacterium (Lactobacillus acidophilus) was found to limit the risk of early premature birth, which researchers hope will lead to new preventative therapies.
A combination of metabolites: glucose, aspartate and calcium, and bacteria; Lactobacillus crispatus and acidophilus, was linked to birth at or before 34 weeks.
Seven different metabolites: leucine, tyrosine, aspartate, lactate, betaine, acetate and calcium, were associated with birth at or before 37 weeks.
Researchers found these links were equally significant when mothers were tested in the first and second trimester.
This suggests those at risk of premature birth could be accurately identified much earlier in pregnancy than current tests allow.
This would then allow them to benefit from medical or surgical treatments that are not possible in late pregnancy.
Researchers also found ethnic differences in the vaginal microbiome.
Black mothers were more likely to have high levels of certain antibacterial proteins: elafin and cathelicidin, linked to cervicovaginal inflammation and therefore premature birth.
However, researchers say the association is not significant enough to explain the increased rates of premature birth among black mothers, and could reflect other environmental and social differences.
Larger studies with more robust data are needed.
Tommy’s chief executive, Jane Brewin, said: “This new study has not only uncovered warning signs that could be used to develop new tests, but also a possible treatment which could make pregnancy safer for the most vulnerable, so this new avenue of research has really exciting potential for clinical practice.”
The study, published in the Journal of Clinical Investigation, was jointly funded by charities Tommy’s and the Rosetrees Trust, as well as the National Institute for Health Research giving a doctoral fellowship to one of the authors and the Wellcome Trust supporting another member of the team.