Preterm labor refers to regular uterine contractions that cause the cervix to open and delivery to occur more than three weeks before your baby's estimated due date.

Babies who are born at less than 37 weeks often have low birth weights (less than 5 1/2 pounds). Their low weights, along with various other problems associated with preterm birth, often puts them at risk of certain health problems.

No one knows exactly what causes preterm labor. In many cases, it occurs among women who have no known risk factors. Care providers and researchers have identified factors that seem to play a role, however.