Physiology of Third Stage of Labor and Placental Separation Signs

Defining the Third Stage of Labor

The third stage of labor begins immediately after the complete delivery of the baby and ends with the successful expulsion of the placenta and fetal membranes. While this phase is relatively short compared to the first two stages, usually lasting between 5 to 30 minutes, it carries a high risk of life-threatening postpartum hemorrhage. During these few minutes, dramatic physiological shifts occur inside the maternal body as the uterus transitions from a muscular organ of expulsion to one of active vessel closure.

Mechanisms of Placental Separation

Once the baby is born, the uterus abruptly shrinks in surface area while maintaining its volume. Because the placenta cannot contract or change shape to match this sudden reduction, it buckles, folds, and shears off its attachment site at the spongy layer of the decidua basalis.

Two distinct separation mechanisms exist:

• Schultze mechanism: Separation begins near the center of the placenta, forming a retroplacental blood clot that pushes the center outward. The placenta descends inside-out, like an inverted umbrella, usually with minimal external bleeding until delivery.
• Duncan mechanism: Separation begins at the lower or lateral edge of the placenta. Blood slips between the membranes and the uterine wall, causing the placenta to slide out sideways like a folded rug, accompanied by early external bleeding.

Signs of Placental Separation

Midwives must carefully observe the mother for reliable physical signs indicating that the placenta has separated from the uterine wall and entered the lower uterine segment or vagina. Waiting for these signs prevents the dangerous practice of pulling on an attached cord.

1. Uterine shape change: The uterus becomes globular, firm, and rises higher in the abdomen, often tilting to one side.
2. Cord lengthening: A lengthening of the visible portion of the umbilical cord outside the vagina indicates the placenta has moved downward.
3. Gush of blood: A sudden trickle or short gush of dark blood often escapes from the vagina as the retroplacental clot is expelled.
4. Suprapubic fullness: The lower abdomen may feel fuller as the displaced placenta sits right above the cervix.

Physiological Control of Uterine Hemorrhage

The mechanism that stops bleeding at the placental site is unique and relies entirely on mechanical pressure rather than clotting alone. When the myometrium contracts forcefully after birth, crisscrossing muscle fibers clamp down tightly on the maternal blood vessels that feed the placenta. These living muscle ligatures act as natural tourniquets. While normal blood clotting factors help seal the raw decidual surface, true hemostasis fails if these muscle fibers relax, making continuous myometrial contraction vital for survival.

Methods of Placental Expulsion

Once the placenta has detached and moved out of the upper uterine cavity, it must be expelled from the body. Midwives manage this using either physiological or active approaches.

• Physiological expulsion: Relying entirely on maternal effort, gravity, and natural contractions without oxytocic drugs or cord traction. The mother pushes gently when she feels the urge.
• Active expulsion: Using prophylactic uterotonics, controlled cord traction with counter-pressure on the uterus, and uterine massage to speed up delivery of the placenta and reduce blood loss.