A typical pregnancy lasts 40 weeks, but for about 1 in 10 pregnant women labor begins too soon. Premature or preterm birth is the leading cause of newborn deaths worldwide. Yet doctors have struggled to understand why preterm birth occurs — or how to predict which pregnancies are at highest risk.

Now, researchers have discovered antibodies that play a role in triggering early labor. The antibodies were present in higher levels and appeared earlier in pregnancy in women who went on to give birth to their babies before 37 weeks’ gestation compared to women who delivered their babies at term.

The research was published in Nature Immunology and could lead to both new ways of screening for preterm birth risk and new interventions to stop it.