The correct sequence of cardinal movements
In a vertex presentation, the fetal head does not simply slide straight down the birth canal. It must adapt its position to the changing shape of the maternal pelvis, and these adaptations are called the cardinal movements of labor. After engagement and descent, the order is flexion, then internal rotation, then extension, and finally external rotation, which is the sequence 4, 2, 1, 3. Restitution occurs just before external rotation, and expulsion of the shoulders and body completes the mechanism. Each movement prepares the head for the next one, so the order follows the anatomy of the pelvis, not chance.
Why flexion comes first
As the head descends and meets resistance from the cervix, the pelvic floor, and the walls of the pelvis, the chin is pressed toward the chest. This flexion changes the leading diameter from the larger occipitofrontal diameter to the smaller suboccipitobregmatic diameter, so the smallest part of the head leads the way. Without good flexion, a larger diameter must pass through the pelvis, and labor tends to be slower. Flexion must therefore happen before the head reaches the level where it needs to rotate.
Internal rotation and extension
The pelvic inlet is widest in its transverse diameter, while the outlet is widest in its anteroposterior diameter. The well-flexed head therefore rotates internally, usually so that the occiput turns anterior toward the symphysis pubis. Only after this rotation can the occiput slip under the pubic arch. The head is then born by extension: the base of the occiput pivots under the symphysis, and the face, nose, mouth, and chin sweep over the perineum. Key point! Extension cannot happen before internal rotation, because an unrotated occiput would not be positioned to pass under the symphysis.
Restitution and external rotation
Once the head is born, it is free of the pelvis and turns back to realign with the shoulders, which are still inside. This untwisting is restitution. The head then turns further as the shoulders rotate internally to lie in the anteroposterior diameter of the outlet, which is seen outside as external rotation. The anterior shoulder is then delivered under the symphysis, followed by the posterior shoulder and the rest of the body. External rotation always follows birth of the head, so it can never come before extension.
How the distractors go wrong
The option 4, 1, 2, 3 places extension before internal rotation, which skips the step that brings the occiput under the symphysis. The option 2, 4, 1, 3 places internal rotation before flexion, even though the smallest diameter must lead before rotation occurs. The option 4, 2, 3, 1 places external rotation before extension, which is impossible because external rotation is a movement of the head after it has already been born. Watch out! Exam items often list the movements out of order; anchor on the idea that rotation inside the pelvis comes before birth of the head, and rotation outside the body comes after it.
Exam takeaway
Memorize the full sequence: engagement, descent, flexion, internal rotation, extension, restitution, external rotation, and expulsion. Flexion makes the head small, internal rotation lines it up with the outlet, extension delivers it, and external rotation realigns it with the shoulders. When the nurse explains this to a student, linking each step to pelvic shape makes the order logical rather than something to memorize blindly.