Preterm labor occurs during pregnancy when the uterus starts to contract before the 37th week of pregnancy, causing the cervix to open (dilate) and thin (efface).

The length of a normal pregnancy is 37 to 42 weeks, measured from the date of the mother's last menstrual period.

Early symptoms of preterm labor often are hard to diagnose. The uterus normally begins to contract around the 20th week of pregnancy. These early contractions (Braxton Hicks contractions) do not dilate or efface the cervix but help prepare the uterus and fetus for labor. These contractions can be mistaken for preterm labor.

Preterm labor is diagnosed in a woman who is 20 to 37 weeks pregnant and has regular uterine contractions. This means 4 or more in 20 minutes, or about 8 or more in 1 hour.

Before 20 weeks, preterm labor that leads to delivery is considered a miscarriage (spontaneous abortion).