The Seven Cardinal Movements and 5 Ps of Labor

Exam Tip: When assessing a patient's "Passenger," remember that "Station" is a measure of descent. A station of 0 indicates that the presenting part is at the level of the ischial spines, also known as engagement.

The Seven Cardinal Movements

As the fetus moves through the birth canal during a vertex delivery, it undergoes a specific sequence of maneuvers to present the smallest possible diameter to the maternal pelvis. A common mnemonic to remember this sequence is: "Every Darn Fetus Is Extremely Eager to Exit".

1. Engagement and Descent

The process begins with Engagement, which occurs when the greatest transverse diameter of the fetal head (the biparietal diameter) passes through the pelvic inlet. This is typically documented at 0 station. Following engagement, the fetus undergoes Descent, a continuous downward movement of the head toward and through the pelvic inlet.

2. Flexion and Internal Rotation

As the fetal head encounters resistance from the cervix or the walls of the pelvis, Flexion occurs. During flexion, the fetal chin is pushed toward the chest, which minimizes the presenting diameter of the head to make passage easier.

Once the head is positioned correctly, it undergoes Internal Rotation. The fetal head rotates approximately $45^{\circ}$ forward to align its long axis with the midline of the mother's pelvis, positioning the head directly under the pubic bone.

3. Extension and External Rotation (Restitution)

As the head moves under the pubic bone, it must undergo Extension. The head extends backward, lifting the chin away from the chest, which allows the head to emerge from the vaginal opening.

Immediately after the head is delivered, the fetus undergoes External Rotation, also known as Restitution. The head rotates externally to face one of the mother's inner thighs. This maneuver is critical because it realigns the fetal shoulders with the pelvic outlet, preparing them for delivery.

4. Expulsion

The final stage is Expulsion. Once the head and both shoulders (the anterior and posterior shoulders) have been delivered, the remainder of the fetal body follows, typically emerging smoothly and rapidly.

Clinical Note: Monitoring the progress of these movements is often done through a Sterile Vaginal Exam (SVE) to assess dilation, effacement, and station. However, an SVE should never be performed on a patient with active vaginal bleeding.

Summary of Labor Mechanics

The successful transit of the fetus depends on the harmony between the Powers (contractions) and the Passenger (the fetus) navigating the Passageway (the pelvis). If these mechanics are disrupted—for example, by fetal malposition (Passenger) or inadequate contractions (Powers)—the labor may become "dysfunctional," requiring interventions such as oxytocin for augmentation or assisted deliveries like vacuum or forceps.