During pregnancy a woman's uterus nurtures and protects the developing fetus during gestation. A full-term pregnancy is approximately 40 weeks. When the fetus has matured and birth is imminent, the baby begins to go through a series of movements that help it navigate through the birth canal.
During labor, the uterus contracts at regular intervals, causing the opening of the uterus, the cervix, to dilate. These contractions are commonly referred to as labor pains. When the contractions cause the cervix to dilate to 10 cm, the opening is large enough to allow the baby to pass from the uterus into the vagina.
The vagina is a muscular tube that can expand to accommodate the baby's head and shoulders. Uterine contractions continue until the baby and the placenta are delivered.