In the first stage of labor, the cervix must thin (efface) and open (dilate) to a diameter of about 10 cm (4 in) in order to allow the fetal head to pass through.
Repeated uterine contractions pushing the fetal presenting part against the inside of the uterus cause the cervix to thin and open over time.
The fetal presenting part is considered to be engaged when the lowest portion is at the level of the ischial spines; this is called the 0 station. Fetal movement down the birth canal is measured by positive stations, using a 0/+3 scale or a 0/+5 scale.
The second stage of labor starts at full dilation of the cervix and is completed when the fetus is delivered. The third stage is the delivery of the placenta.