5.3 Cardinal Movements, Third & Fourth Stages of Labor

The Cardinal Movements of Labor

Cardinal Movements in Vertex Presentation

To navigate the maternal pelvis—an irregular curved conduit with an oval transverse inlet and an elongated anteroposterior outlet—the fetal head must execute a synchronized series of seven positional adaptations:

1. Engagement: The biparietal diameter (the greatest transverse cephalic diameter, measuring approximately 9.5 cm) passes through the maternal pelvic inlet. Clinically, the leading bony edge of the fetal occiput reaches the level of the maternal ischial spines (station 0). In primigravidas, engagement often occurs 2 to 4 weeks prior to labor; in multiparas, it frequently occurs at the onset of active labor.
2. Descent: The continuous downward movement of the presenting part through the pelvic architecture. Propelled by four forces: (a) downward hydrostatic pressure of amniotic fluid, (b) direct fundal pressure of the contracting uterus on the fetal breech, (c) maternal abdominal bearing-down efforts, and (d) active extension and straightening of the fetal vertebral column.
3. Flexion: As the descending head meets passive mechanical resistance from the cervix, pelvic sidewalls, and pelvic floor musculature (levator ani), the fetal head flexes until the chin rests firmly on the sternum. This mechanical shift substitutes the smaller suboccipitobregmatic diameter (9.5 cm) for the substantially larger occipitofrontal diameter (11.5 cm), enabling passage through the midpelvis.
4. Internal Rotation: The fetal head, which entered the pelvic inlet in an occiput transverse (OT) or occiput oblique position, rotates 45° to 90° internally as it reaches the gutter-like slope of the levator ani muscles. The occiput rotates anteriorly toward the maternal symphysis pubis (Occiput Anterior / OA), aligning the long anteroposterior diameter of the fetal head with the longest diameter of the pelvic outlet.
5. Extension: As the sharply flexed fetal head descends to the vulvar outlet, it encounters resistance from the upward muscular pull of the pelvic floor and the rigid subpubic arch. The occiput pivots directly beneath the inferior border of the symphysis pubis. The head extends upward and outward, allowing the occiput, bregma, forehead, nose, mouth, and chin to emerge sequentially over the perineum.
6. Restitution and External Rotation: Once the head is born, it spontaneously untwists 45° (restitution) to realign perpendicularly with the shoulders in the midpelvis. As the fetal shoulders subsequently undergo internal rotation within the maternal pelvis to align their biacromial diameter in the anteroposterior pelvic diameter, the externally delivered head rotates another 45° (external rotation, 90° total from delivery position) so the fetal face directly faces the maternal right or left medial thigh.
7. Expulsion: Following external rotation, the anterior shoulder descends beneath the symphysis pubis and delivers with gentle downward traction. The clinician then elevates the fetal head upward toward the maternal abdomen to deliver the posterior shoulder smoothly over the perineal body. The remainder of the fetal trunk and lower extremities expels rapidly without resistance.

The Third Stage of Labor

Third Stage of Labor (Delivery of Infant to Complete Placental Expulsion)

The normal duration of the third stage is between 5 and 30 minutes. Following delivery of the infant, the sudden reduction in uterine volume shears the inelastic placenta away from the contracting decidual basalis.

Classical Signs of Placental Separation

1. Sudden gush of blood from the vaginal introitus.
2. Lengthening of the umbilical cord as the placenta descends into the lower uterine segment and upper vagina.
3. Change in uterine shape from discoid to a firm, globular configuration, with the fundus rising in the abdomen.

Active Management of the Third Stage of Labor (AMTSL)

Active management is the global gold standard recommended by the International Federation of Gynecology and Obstetrics (FIGO), the World Health Organization (WHO), and ACOG to prevent postpartum hemorrhage:

• Prophylactic Uterotonic: Oxytocin 10 IU intramuscularly (IM) or as a diluted intravenous infusion (20–40 IU in 1,000 mL crystalloid at 125–250 mL/hr) administered immediately after the delivery of the anterior shoulder or neonate. Oxytocin is the first-line agent of choice due to its rapid onset and absence of hypertensive side effects.
• Controlled Cord Traction (Brandt-Andrews Maneuver): Gentle, steady downward traction applied to the clamped umbilical cord with one hand, while the other hand is placed suprapubically to provide firm counter-traction upward on the lower uterine segment. This prevents uterine inversion—a catastrophic complication caused by pulling on an unseparated placenta with a relaxed fundus.
• Fundal Uterine Massage: Immediate vigorous palpation and massage of the uterine fundus following placental delivery until myometrial tone is firmly contracted.

Caution: Never apply aggressive traction on an umbilical cord without active suprapubic counter-traction, and never pull on the cord when the uterus is atonic, as this is the primary iatrogenic trigger for acute uterine inversion.

The Fourth Stage of Labor

Fourth Stage of Labor (Immediate Hemodynamic Recovery: Hours 1–2 Postpartum)

The fourth stage spans the first 1 to 2 hours following complete placental expulsion. During this window, profound physiological adaptations occur:

• Hemodynamic Autotransfusion: Uterine contraction and relief of inferior vena cava compression rapidly shifts approximately 500 mL of blood from the uteroplacental bed into the maternal central circulation. Maternal stroke volume and cardiac output transiently spike by 60% to 80% before stabilizing, often precipitating transient physiological bradycardia.
• Midwifery Assessment Protocol: Vital signs (blood pressure, pulse, respirations), uterine fundal height and firmness (the fundus should be firm, midline, and at or below the umbilicus), lochial flow (rubra, moderate, no clots larger than a golf ball), perineal integrity, and bladder distension are evaluated every 15 minutes for the first hour, and every 30 minutes for the second hour.
• Bladder Vigilance: A full, distended bladder displaces the uterus upward and laterally (frequently to the right), preventing effective myometrial contraction and serving as the leading functional cause of early secondary uterine atony and postpartum hemorrhage.