A test that can predict whether a woman is likely to give preterm birth has been identified by Swedish researchers.
According to the scientists, high levels of lactate in vaginal fluid is strongly linked to onset of labour within 48 hours.
UK experts said that the test can help to plan care and reassure women whose membranes rupture prematurely.
The study has been published in Obstetrics and Gynaecology journal, BJOG.
The tool, which is known as the "Lac-test", was reviewed in 86 women with singleton pregnancies of 20 to 36 weeks gestation.
Among 23 women with high lactate concentrations, 87 percent had spontaneous onset of labour within 48 hours. In 58 women with low lactate concentrations - in effect a negative Lac-test - only 5 percent went into labour within 48 hours.
The median time between examination and onset of labour was 13.6 hours for those with a high lactate concentration and 48 days for those with a low lactate value, reports BBC.
According to the study's experts, earlier studies had found an association between high lactate concentration in vaginal fluids and rupture of membranes in pregnancies of more than 34 weeks gestation, but this is the first time the link has been seen under 34 weeks.
Being able to predict labour is even more valuable in these premature babies, as steroids can be given to promote lung development and women can be referred to specialist hospitals.
Study leader, Dr Eva Wiberg-Itzel, from the Department of Clinical Science and Education at the Karolinska Institute, said: "The diagnosis of ruptured membranes is easy when there is an obvious leakage of amniotic fluid, but more difficult when the leak is scanty or intermittent.
"We believe that the Lac-test adds important information in clinical practice."
Professor Philip Steer, editor-in-chief of BJOG, said it seemed a promising tool to predict the onset of labour.
Steer said: "A more reliable diagnosis of PPROM could help doctors determine when to keep women in hospital, and improve the timing of antenatal steroid therapy."