Physiology and Signs of Second Stage of Labor

Defining the Second Stage: What Marks the Transition

The second stage of labor begins when the cervix reaches full dilatation at ten centimeters and ends with the complete delivery of the baby. This is the expulsive stage, where uterine contractions work alongside maternal pushing efforts to move the fetus through the birth canal. A defining feature of this transition is the shift from passive cervical dilatation to active fetal expulsion. Midwives recognize this stage clinically through vaginal examination showing a fully dilated cervix, or sometimes definitively when the parturient experiences an uncontrollable urge to bear down as the fetal head descends onto the pelvic floor.

Physiology and Forces of Expulsion

Expulsion relies on two primary forces: involuntary uterine contractions and voluntary maternal abdominal effort. Uterine contractions become stronger, lasting longer and occurring more frequently, driven by the release of oxytocin. As the myometrium contracts, it exerts downward pressure on the fetus. When the presenting part reaches the pelvic floor, it triggers Ferguson's reflex, sending nerve impulses to the posterior pituitary to release more endogenous oxytocin. This natural surge compels the mother to push, adding intra-abdominal pressure from the diaphragm and abdominal wall muscles to complete the expulsion process.

Signs Indicating Full Dilatation and Descent

Midwives look for both subjective and objective indicators to confirm the second stage.

• Involuntary bearing down: The mother reports feeling an intense, uncontrollable urge to push with contractions.
• Change in contraction character: Contractions may become more frequent, lasting up to ninety seconds.
• Perineal bulging: The perineum stretches and bulges outward as the fetal head descends.
• Anal sphincter dilitation: The anal sphincter may relax, and the-ore external rectal mucosa can become visible.
• Show: An increase in blood-tinged mucus, often called the bloody show.
• Visual confirmation: The fetal head becomes visible at the vaginal introitus, crowning between contractions.

Mechanisms of Labor: Cardinal Movements

As the fetus travels through the maternal pelvis, it performs a series of passive movements known as the cardinal movements of labor. These positional adjustments allow the irregular shape of the fetal head to fit through the varying diameters of the pelvis.

1. Engagement: The widest part of the fetal head passes through the pelvic inlet.
2. Descent: The downward movement of the fetus through the birth canal.
3. Flexion: The fetal chin tucks tightly against the chest to present the smallest head diameter.
4. Internal rotation: The head rotates so the occiput moves toward the front of the pelvis.
5. Extension: The fetal head pivots upward under the pubic bone, emerging from the vagina.
6. Restitution: The head turns back to align naturally with the shoulders after delivery.
7. External rotation: The shoulders rotate inside the pelvis to pass through the pelvic outlet.

Monitoring Maternal and Fetal Well-being During Expulsion

During the second stage, close monitoring ensures both mother and baby tolerate the physical stress of expulsive efforts. The midwife assesses fetal well-being by listening to the fetal heart rate immediately after a contraction using a Pinard stethoscope or Doppler ultrasound. Prolonged pushing without adequate rest can cause fetal hypoxia. For the mother, the midwife tracks vital signs, observes her emotional coping mechanisms, and monitors urine output. Assessing the color of the amniotic fluid when membranes rupture is also critical, as thick meconium staining demands immediate readiness for neonatal resuscitation.