The cardinal movements of labor refer to the changes in position of the fetal head during its descent through the birth canal in vertex presentation:

• Descent (lightening): movement of the fetal head through the pelvis toward the pelvic floor. The highest rate of descent occurs during the deceleration phase of the first stage and during the second stage of labor.

• Engagement: the descent of the widest diameter of the presenting fetal part below the plane of the pelvic inlet. The widest diameter in cephalic presentation is the biparietal diameter. In breech presentation, the bitrochanteric diameter determines the station.

• Flexion: a passive movement that permits the smallest diameter of the fetal head (suboccipitobregmatic diameter) to pass through the maternal pelvis

• Internal rotation: The fetal occiput rotates from its original position (usually transverse) toward the symphysis pubis (occiput anterior) or, less commonly, toward the hollow of the sacrum (occiput posterior).

• Extension: The fetal head is delivered by extension from the flexed position as it travels beneath the symphysis pubis.

• External rotation: The fetal head turns to realign with the long axis of the spine, allowing the shoulders to align in the anterior-posterior axis.

• Expulsion: The anterior shoulder descends to the level of the symphysis pubis. After the shoulder is delivered under the symphysis pubis, the remainder of the fetus is delivered.