Identifying mothers is vital
DNA differences which appear to affect the risk of giving birth early have been found by US scientists.
The US National Institutes of Health study found the variants in both babies and mothers, a US conference was told.
It is thought they may play a role in controlling immune responses which could theoretically trigger labour if they become too powerful.
Premature birth - which accounts for 7% of UK births - is one of the biggest threats to a baby's future health.
The causes of premature birth are poorly understood, although infections and other medical complications are blamed in some cases.
The study looked at 700 DNA variants in 190 genes in women who delivered early, and those who carried their baby to term.
The cord blood of the babies was also tested for these variations.
They narrowed the search down to a handful of gene variations found more often in the women who gave birth prematurely, and their babies.
In particular, babies who carried a DNA variant in the gene for the "Interleukin 6 receptor" were more likely to be born early.
This was a good candidate gene because Interleukin 6 is produced by cells in response to infection and is involved in inflammation.
High levels of Interleukin 6 in the amniotic fluid and foetal blood have been linked to the onset of premature labour.
Dr Roberto Romero, who led the study, said: "Our hypothesis is that the mother and/or the foetus signal the onset of preterm labour when the environment inside the uterus is unfavourable and threatens the survival of the maternal-foetal pair.
"When there is an infection in the uterus, the onset of premature labour appears to have survival value - it would allow the mother to rid herself of infected tissue and preserve her ability to have future pregnancies."
The chief executive of charity Bliss, Andy Cole, welcomed the study results.
"In England alone, 54,000 babies are born prematurely each year, a third of these for no known reason," he said.
"The development of a reliable test for identifying these mothers is vital in ensuring our most vulnerable babies have the best possible outcomes."