The risk of a premature birth could be assessed just 10 weeks into a woman's pregnancy, research suggests.
Around four in 50 babies born in the UK are delivered before 37 weeks, putting the infants at risk of infections, asthma and feeding problems, as well as chronic health issues in later life.
Why premature births occur is often unclear, but they have been linked to infections and inflammation in the cervix, the neck of the womb.
A team from the charity Tommy's has now found certain bacteria and chemicals in pregnant women's cervical-vaginal fluid may act as a warning sign.
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Identifying at-risk women early may allow medics to administer treatments that extend their pregnancy. These bacteria and chemicals could even one day be manipulated to create a new pregnancy-extending therapy.
"With 60,000 babies born prematurely each year in the UK, there's a real and urgent need for better ways to predict and prevent preterm birth," said Jane Brewin, chief executive of Tommy's.
"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."
Infections and inflammation are known to shorten and weaken the cervix, which normally lengthens and produces chemicals to protect a foetus, shortening and softening in preparation for labour.
To better understand why this shortening and softening may happen too soon, the scientists analysed 346 mothers from four UK hospitals, of whom 60 gave birth prematurely.
Cervical-vaginal fluid samples were taken 10 to 15 weeks into their pregnancy and again at 16 to 23 weeks.
The samples were grouped according to their make-up of microorganisms and molecules that are naturally created by the body, like sugars and amino acids.
These were compared against the women's cervical length measurements, the standard NHS assessment for premature birth risk. The women were then followed up to determine who gave birth before 37 weeks.
Results – published in the Journal of Clinical Investigation – reveal a combination of the molecules glucose, calcium and the amino acid aspartate with the bacteria Lactobacillus crispatus and L. acidophilus were linked to a woman giving birth at or before 34 weeks.
Seven molecules – leucine, tyrosine, aspartate, lactate, betaine, acetate and calcium – were associated with going into labour at 37 weeks or earlier.
In contrast, L. acidophilus alone were found to limit the risk of an early birth, raising hopes this could be utilised to ward off premature births.
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"With so many factors in play, it's unlikely testing for the same single bacteria species will predict preterm birth in every mother, but we now have a panel of bacteria and metabolites that could be useful," said lead author Professor Rachel Tribe, from King's College London.
"In particular, tests during early pregnancy for L. crispatus and L. acidophilus could provide reassurance to mothers who would otherwise be unduly worried and help those who need it get specialist care as soon as possible."
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All of the aforementioned bacteria and molecules were found to influence a woman's premature birth risk regardless of whether she was tested in her first or second trimester, which may help to identify vulnerable individuals earlier than existing tests.
These women may then benefit from drugs or surgery that are not possible later on in pregnancy. A small hormonal pill can be placed in the vagina or the cervix can be stitched, strengthening it.
"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," said co-author Professor Andrew Shennan OBE, from King's College London.
"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."
Foetal fibronectin is a protein that is thought to keep the amniotic sac, which cushions a developing baby, attached to the uterus's lining. If the attachment is disrupted, due to an infection or inflammation, foetal fibronectin can be released into cervical secretions.
"The sooner we can find out who's at risk, the more we can do to keep mothers and babies safe," said Professor Shennan.
The results also suggest that Black women have higher levels of certain antibacterial proteins that are linked to cervical-vaginal inflammation and a subsequent premature birth risk.
Larger studies are required to investigate this further, according to the scientists.