Researchers at University College London and King's College London have developed a simple saliva test that may help detect which expectant women are likely to go into premature labour.
Researchers believe that early identification would allow mothers to be given steroids which help in the development of the baby's lungs, preventing disability and death.
The study appears in the British Journal of Obstetrics and Gynaecology, reports The BBC.
Progesterone plays a central role in helping a woman carry babies to term by stopping the womb from contracting.
In the research, scientists studied 92 women deemed to be at increased risk of having a preterm birth, and found that the women who delivered spontaneously before 34 weeks had much lower salivary levels of progesterone than those giving birth at term, after 37 weeks.
This measurable difference in progesterone was apparent at all gestational ages from 24 weeks onwards.
Lead author Professor Lucilla Poston, from the Maternal and Foetal Research Unit at King's College London, said: "Saliva is easy to collect, there is no need for a needle or a blood sample and it would be wonderful if in the future we only had to ask a pregnant woman to produce a small sample of saliva to know whether or not she was at risk of very early premature birth."