Physiological labor

Why does labor begin?

The onset of labor is the result of the synchronized work of the nervous, humoral, and fetoplacental systems. The triggering mechanism is fetal maturity: the fetal hypothalamo-pituitary system stimulates the adrenal glands to release cortisol.

Simultaneously, global systemic changes occur in the mother's body:

• Estrogen dominance: placental estriol prepares the myometrium by increasing the synthesis of ATP and glycogen and enhancing membrane permeability to calcium ions.
• Oxytocin and prostaglandins: the pituitary gland secretes oxytocin, while prostaglandins (E2 and F2α) are synthesized in the fetal membranes, directly triggering uterine contractions.
• Nervous regulation: inhibitory processes in the cerebral cortex are enhanced while subcortical structures become excited, ensuring the reflex automaship of the uterus.
• Immune shift: a decrease in the mother's melatonin levels suppresses immunosuppressants, stimulating the rejection of the fetus as an allograft.

Preparation and engagement of the head

To successfully pass through the birth canal, the fetal head must adapt to the dimensions of the pelvis. This occurs through molding — the overlapping of skull bones in a shingle-like fashion due to the pliability of the sutures and fontanelles.

Before labor, the head presses against the pelvic inlet. Normally, the sagittal suture is positioned at an equal distance between the pubic symphysis and the sacral promontory — this is synclitic (axial) engagement. A slight anterior asynclitism (displacement of the suture closer to the promontory) is acceptable. The lowest point of the presenting part is called the leading point — it is precisely here, due to tissue edema, that the caput succedaneum forms (caput succedaneum).

Biomechanism in the anterior occiput presentation

This is the classic and most frequent variant of normal labor. It includes 4 synchronous moments inextricably linked with the progressive movement of the fetus:

1. Flexion of the head. According to the unequal lever rule, intrauterine pressure acts more strongly on the short arm (the occiput). The chin is pressed against the chest, and the small fontanelle becomes the leading point. The head reduces its size to the suboccipitobregmatic diameter (9.5 cm).
2. Internal rotation of the head. Passing from the wide part of the pelvis to the narrow part, the head encounters varying resistance from the bones. It rotates, setting the sagittal suture in the anteroposterior diameter of the pelvic outlet (with the occiput toward the symphysis).
3. Extension of the head. The suboccipital fossa rests against the lower edge of the pubic symphysis, forming the pivot point (hypomochlion). Around this point, the head extends and is born (vertex, forehead, face, chin).
4. Internal rotation of the trunk and external rotation of the head. The shoulders rotate into the anteroposterior diameter. The delivered head rotates to face the mother's thigh. The anterior shoulder is born first (fixing under the symphysis), followed by the posterior shoulder.

Features of the occiput posterior position

By the end of the first stage of labor, about 35% of fetuses are in the posterior position, but most of them rotate by 135° and are born in the anterior position. If this does not occur (in about 1% of cases), the biomechanism changes and consists of 5 movements.

The main difference is the need for maximal additional flexion (third movement). The fetal head forms the first point of fixation (anterior margin of the large fontanelle at the symphysis), flexes for the birth of the occiput, and then forms the second point of fixation (suboccipital fossa at the coccyx) for final extension. Clinically, this manifests as a prolonged second stage of labor and the formation of a dolichocephalic head shape with a caput succedaneum in the area of the large fontanelle.

Class and exam questions

What else to study on this topic

• Topography of fontanelles and sutures during vaginal examination in the dynamics of labor
• Morphology and layered structure of the caput succedaneum
• Role of prostaglandins E2 and F2α in the latent and active phases of labor
• Mechanism of delivery of the anterior and posterior shoulders (points of fixation)
• Diagnostic errors in assessing the station of the fetal head in occiput posterior position

Other topics in the "Obstetrics" section

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