Uterine contractions

Bearing-down efforts and biomechanics of fetal expulsion

At the end of the second stage of labor, involuntary uterine contractions are joined by bearing-down efforts (pushing) — reflex acts aimed at expelling the fetus. Their activation is explained by the Ferguson reflex: when the presenting part of the fetus descends and presses on the pelvic nerve plexuses, the receptors of the cervix, and the perineal muscles, the parturient woman experiences an irresistible urge to push.

The mechanics of the process consist of breath-holding during exhalation combined with a powerful contraction of the diaphragm and the muscles of the anterior abdominal wall. This leads to a sharp spike in intra-abdominal and intrauterine pressure. Since the uterus is securely fixed in the pelvis by the broad, round, and uterosacral ligaments, all the energy of the generated pressure is directed exclusively toward pushing the child out.

The fetus advances along the path of least resistance, following the axis of the pelvis. It descends onto the pelvic floor, stretches the vulvar cleft, and is born. The expulsion stage ends with the complete birth of the newborn and the discharge of the posterior portion of amniotic fluid.

Placental contractions and the third stage of labor

Immediately after the child is born, the third stage of labor (placental stage) begins, the essence of which is the detachment and expulsion of the placental complex (placenta, fetal membranes, and umbilical cord). Normally, this phase takes from 5 to 30 minutes.

The main driving force of this stage is placental contractions — rhythmic contractions of the myometrium. Several key factors contribute to placental separation:

• A sharp decrease in the volume of the uterine cavity after the fetus has left it.
• The absence of contractile capacity in the placental tissue itself (the myometrium contracts, whereas the placenta does not, which causes tissue displacement).
• The location of the placenta within the functional layer of the mucous membrane, which guarantees its unhindered detachment from the basal layer.

Mechanisms of placental separation

In obstetrics, two classic variants of placental detachment from the uterine wall are distinguished.

Central separation (Schultze method) This occurs most frequently. Separation starts strictly from the central part of the placenta. Under the influence of muscle contractions, the placenta rises in the form of a mound. The uteroplacental vessels rupture, and blood accumulates between the uterine wall and the placenta, forming a retroplacental hematoma. It mechanically pushes the placenta into the cavity, accelerating further separation. As a result, the placenta is delivered turned inside out: the fetal surface, covered with the amniotic membrane, faces outward.

Peripheral separation (Duncan method) Separation begins at the edge. The main difference is the absence of a retroplacental hematoma. Blood from damaged vessels does not accumulate, but immediately flows down freely, concurrently stripping the fetal membranes. With each new placental contraction, successive areas detach. Under the weight of its own mass, the detached edge sags and pulls the rest along with it. From the birth canal, such a placenta emerges folded like a cigar, with the maternal surface facing outward.

Blood loss and mechanisms of hemostasis

The separation of the placenta is naturally accompanied by bleeding from the exposed placental site. The physiological norm is considered to be a loss of no more than 0.5% of the body weight of the parturient woman (in absolute terms, this is approximately 300–500 ml).

The cessation of bleeding after the birth of the placenta is a critically important process ensured by three mechanisms:

1. Myometrial contraction. The intensive work of the uterine musculature deforms, twists, sharply bends, and displaces blood vessels, narrowing the terminal segments of the arteries.
2. Specific vascular anatomy. Uterine arteries have a spiral structure. When the uterus contracts, they are drawn in and displaced into the deeper layers of the muscular wall, where they undergo powerful compression from the contracting fibers. This is the most important factor in the final cessation of bleeding.
3. Local hemostasis. In the area of the placental site, the blood coagulation system is actively triggered, reliably thrombosing the damaged areas.

Class and exam questions

What else to study on this topic

• Anatomy of the uterine ligamentous apparatus and its biomechanical role in childbirth
• Pelvic axis and trajectory of fetal movement during the expulsion stage
• Structure of the placenta: placental disk, fetal membranes, umbilical cord
• Spiral arteries of the uterus and the mechanics of vascular hemostasis
• Activation of the blood coagulation system in the area of the placental site

Other topics in the "Obstetrics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.