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How do uterine contractions open the cervix during the first stage of labor?

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A series of medical illustrations showing the stages of childbirth: the baby's position at the onset of labor, flexion, internal rotation of the head, extension, external rotation of the head, and the uterus immediately after birth with the placenta and umbilical cord. — birth Sequential changes in the position of the child during labour.

During the first stage of labor, uterine contractions facilitate the dilatation (opening) of the cervix through a twofold mechanical process.1

First, because the uterus is a muscular organ containing the fluid-filled amnion and the fetus, its contractions compress these contents. Since the contents are incompressible, they are forced toward the isthmus—the upper opening of the cervix—acting like a wedge that pushes further into the opening with each contraction.1

Second, the muscle fibers of the uterine walls are attached to the neck of the uterus. As these fibers shorten during a contraction, they pull the cervical tissues upward and away from the opening, actively enlarging it.1

This combined action forces the amnion and fetus downward against the cervix while simultaneously pulling the resisting cervical walls upward over the advancing fetus. While the hormone oxytocin is known to stimulate these uterine contractions, it is considered a facilitator of the process rather than an essential requirement for labor to occur.12