Around 2% of pregnant women have premature rupture of membranes
Researchers have identified a test which can predict whether a woman is likely to give birth if her waters break early in pregnancy.
High levels of lactate in vaginal fluid is strongly associated with onset of labour within 48 hours, according to Swedish researchers.
UK experts said the test would help to plan care and reassure women whose membranes rupture prematurely.
The study appears in Obstetrics and Gynaecology journal, BJOG.
Known as the "Lac-test", the tool was assessed in 86 women with singleton pregnancies of 20 to 36 weeks gestation.
Among 23 women with high lactate concentrations, 87% had spontaneous onset of labour within 48 hours.
In 58 women with low lactate concentrations - in effect a negative Lac-test - only 5% 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.
The researchers said previous 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.
Preterm prelabour rupture of membranes (PPROM) - waters breaking in pregnant women before 37 weeks - occurs in 2% of pregnancies and accounts for one-third of all preterm births.
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.
"A more reliable diagnosis of PPROM could help doctors determine when to keep women in hospital, and improve the timing of antenatal steroid therapy."
Professor Andrew Shennan, an obstetrician at St Thomas' Hospital in London, said the test could solve a potentially important problem.
"When membranes rupture unexpectedly there's a big dilemma - do you deliver the baby or not, will this woman go into labour or not?
"We would probably give steroids anyway as it's a high risk situation, but the main thing is being able to tell the women what's likely to happen.
"We can also get them to a specialist unit if needed."