Dilation, station and effacement are the 3 key things to look at when you are determining how far into a labour a woman has progressed.
Cervical dilation is the opening of the cervix, the entrance to the uterus during childbirth. Cervical dilation may occur naturally, or may be induced. The cervix must be 10cm to birth a baby.
The fetal station is a measurement of how far the baby has descended in the pelvis, measured by the relationship of the fetal head to the ischial spines. A baby is fully engaged at 0 station and crowning at +5 station.
Cervical effacement refers to a thinning of the cervix. It is a component of the Bishop score. It can be expressed as a percentage. Prior to effacement, the cervix is like a long bottleneck, usually about four centimeters in length. Throughout pregnancy, the cervix is tightly closed and protected by a plug of mucus. The cervix must be 100% effaced to birth a baby.