Labor is a series of rhythmic, progressive contractions of the uterus that gradually move the fetus through the lower part of the uterus (cervix) and birth canal (vagina) to the outside world.
(See also Overview of Labor and Delivery.)
Labor occurs in three main stages:
First stage: This stage (which has two phases: initial and active) is labor proper. Contractions cause the cervix to open gradually (dilate) and to thin and pull back (efface) until it merges with the rest of the uterus. These changes enable the fetus to pass into the vagina.
Second stage: The baby is delivered.
Third stage: The placenta is delivered.
Labor usually starts within 2 weeks of (before or after) the estimated date of delivery. Exactly what causes labor to start is unknown. Toward the end of pregnancy (after 36 weeks), a doctor examines the cervix to try to predict when labor will start.
On average, labor lasts 12 to 18 hours in a woman’s first pregnancy and tends to be shorter, averaging 6 to 8 hours, in subsequent pregnancies. Standing and walking during the first stage of labor can shorten it by more than 1 hour.