The role of the fetus and fetoplacental complex in the initiation of labor
The initiation of the labor process begins when the fetal hypothalamic-pituitary system reaches maturity. At this point, the hypothalamus secretes corticotropin-releasing hormone, which activates the anterior pituitary and the fetal adrenal glands. The adrenal glands begin to secrete dehydroepiandrosterone sulfate and cortisol, while the pineal gland produces melatonin.
At the fetoplacental complex level, these components are converted into active substances:
- Melatonin stimulates the synthesis of placental estriol.
- Fetal cortisol and estriol trigger the production of placental oxytocin.
- Cortisol activates the enzyme phospholipase A2, which releases arachidonic acid from membrane phospholipids, and cyclooxygenase converts it into prostaglandins PGE2 and PGF2α.
Maternal mechanisms and uterine contractile activity
Within the maternal organism, complete readiness of the myometrium for contractions is established. Placental estriol promotes the formation of intercellular gap junctions and increases the density of receptors for oxytocin and prostaglandins.
The posterior lobe of the maternal pituitary gland releases oxytocin, which, together with placental compounds, stimulates the production of uteroplacental prostaglandin PGF2α. The effect of these uterotonics on myometrial receptors induces contractions.
Neuromuscular regulation of contractions is provided by the autonomic system:
- With maximal excitation of the sympathetic division, norepinephrine is released.
- In response, the parasympathetic center is activated, and under the action of acetylcholine, the circular muscles of the uterus contract while the longitudinal muscles simultaneously relax.
- Between contractions, a pause occurs for complete relaxation and restoration of myometrial contractile proteins.
Prodromal signs of labor and assessment of cervical readiness
Before the onset of labor, the body exhibits a number of characteristic signals. The main prodromal signs include:
- Dropping of the abdomen 2–3 weeks before labor due to engagement of the presenting part and stretching of the lower segment.
- Weight loss of 1–2 kg a few days prior.
- Mood swings, increased excitability, or apathy.
- Decreased fetal motor activity and the appearance of preliminary pains (false labor).
- Loss of the mucous plug, often with minor bloody discharge.
- Cervical ripening, the degree of which is assessed using the modified Bishop score without accounting for the relation of the fetal head to the pelvic planes.
The fetus triggers labor via cortisol and estriol, the placenta adds prostaglandins, and the uterus contracts under the influence of oxytocin.
Class and exam questions
What role do the fetal adrenal glands play at the onset of labor?
The fetal adrenal glands secrete dehydroepiandrosterone sulfate (the precursor of placental estriol) and cortisol, which activates phospholipase A2 and cyclooxygenase for prostaglandin synthesis.
What does the modified Bishop score assess?
It assesses the consistency, length, patency of the cervical canal, and its position relative to the pelvic axis, without taking into account the relation of the head to the pelvic planes.
Which Bishop scores correspond to a mature cervix?
A mature cervix corresponds to scores from 5 to 8 points. A score of 0–2 points indicates an unripe cervix, and 3–4 points indicate an insufficiently ripening cervix.
What else to study on this topic
- The synthesis cascade of prostaglandins PGE2 and PGF2α via phospholipase A2
- Features of autonomic nervous regulation and the function of the longitudinal uterine muscles
- Morphological changes in collagen and elastin during cervical ripening
- Differences between the classical and modified Bishop scores
- Clinical significance of preliminary pains and loss of the mucous plug
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