Which cardinal movement allows the fetus's shoulders to pass through the pelvic inlet?

The cardinal movement that allows the fetus's shoulders to pass through the pelvic inlet is internal rotation. This movement occurs after the fetal head descends and engages with the pelvic inlet.

During labor, once the fetal head rotates to align with the pelvic axis, the shoulders also need to adjust in position to fit through the narrowest part of the birth canal. The internal rotation is crucial as it enables the shoulders to align with the pelvis in a position that allows them to navigate through the inlet effectively. This marked change in the orientation of the fetus's body is essential for facilitating the subsequent stages of labor and delivery.

While other movements like engagement, descent, and flexion are important in the overall process of labor and the mechanics of childbirth, they each serve different roles in the positioning and movement of the fetus through the birth canal. Internal rotation is specifically the movement that permits the shoulders to pass after the head has successfully navigated the pelvic inlet.