Methods
The study was conducted at five tertiary care university obstetric centers serving diverse populations in North America. The protocol was approved by the human subjects review board at each institution. Women who were seen for unscheduled obstetric care between 24 and 34 weeks’ gestation with complaints suggestive of preterm labor (e.g., suspected amniorrhexis, uterine activity, abdominal discomfort, change in vaginal discharge, bleeding, or cramping) were screening for eligibility. Gestational age was estabished by date of last menstrual period and confirmed by ultrasonography or was set by ultrasonography if menstrual dates were unreliable or discordant by > 10 days.