The incidence of premature delivery (at less than 37 completed weeks) has been stable at between 5–10% of deliveries over the past 20 years despite intensive research in this field. Preterm delivery is a major cause of perinatal morbidity and mortality. The identification of women at increased risk for this condition has traditionally been based on clinical history (e.g. previous premature birth or second-trimester miscarriage), digital evaluation of the cervix and the occurrence of uterine contractions.