Delivery before 37 weeks is called preterm or premature. If preterm labor leads to a preterm delivery, your baby may be at risk for health problems. Your uterus prepares for delivery by tightening and relaxing during your pregnancy. These are contractions and most women feel them from time to time during their pregnancy. Occasional contractions that do not cause the cervix to change are not preterm labor.
Labor occurs when contractions are frequent or intense enough to cause cervical changes.
The cervix is the opening to your uterus. Your cervix should stay closed until you approach your due date. This keeps your baby safely inside your uterus until it's mature enough to survive outside your womb. The early signs of labor can be subtle and hard to detect. Many women are unaware of their uterus tightening.