Obstetrics and Gynecology - A. M. Gromova 2000

Physiology of Labor
Regulation of Labor Activity

Childbirth is a complex, multi-stage physiological process that is triggered and completed through the interaction of numerous body systems.

The regulation of uterine motor function in a pregnant woman's body is carried out via neural and humoral pathways. The Central Nervous system plays a decisive role in establishing optimal conditions for the normal development and course of Pregnancy and labor. The onset and progression of labor activity are rooted in unconditioned Reflexes. These reflexes originate in the uterine receptors, which receive stimuli from the conceptus and the fetus. The Uterus serves as a receptive field through which the maternal Organism interacts with the fetus. Furthermore, the chemo-, baro-, thermo-, and mechanoreceptors of the uterus, by receiving stimuli from the conceptus, play a major role in modulating the function of the maternal organism. Changes in the information transmitted from the fetus to the pregnant woman lead to reflex alterations in uterine tone and motility. The onset of labor should be viewed as the result of a gradual integration of morphological, hormonal, biochemical, and biophysical states.

During labor, The stimulation of receptors not only in the uterus but also in the birth canal is of great significance. As new receptors become involved in the process, the strength and frequency of uterine contractions change, subsequently joined by the contractions of striated Muscles (pushing efforts).

The Nature and degree of various reflex responses depend not only on The Effect of humoral and hormonal factors on The Nervous System, but also on the tone of the sympathetic and parasympathetic Divisions of the Autonomic nervous system. The uterus is innervated by sympathetic (adrenergic) and parasympathetic (cholinergic) nerves. The sympathoadrenal system plays a major role in regulating Homeostasis across various functional states of the body. During pregnancy, The activity of the sympathetic nervous system predominates, with the release of adrenaline into the humoral environment, whereas during labor, the activity of the parasympathetic nervous system increases—that is, the activity of the cholinergic system rises with the release of acetylcholine into the parturient woman's humoral environment, to which the myometrium responds with rhythmic contractions. Adrenaline, noradrenaline, and its precursor dopamine cause depolarization of myometrial Cells, accelerate Action Potential discharges and uterine contractions, and conversely, can relax the uterus by suppressing typical activity and Cell membrane hyperpolarization. The Physiological Effect of catecholamines is exerted via Two Types of adrenoceptors: a and ß. Stimulation of a-adrenoceptors leads to contraction of the uterine muscles, while stimulation of β₂-adrenoceptors results in the inhibition of uterine contractile activity.

Throughout labor, various fluctuations in the tone of one or the other branch of the autonomic nervous system are observed. During contractions, the uterus is subjected to the Influence of the cholinergic system.

In physiological labor, under the action of a specific amount of the mediator acetylcholine, contractions of the uterine muscles occur; The breakdown of acetylcholine by cholinesterase is accompanied by a gradual subsidence of the contraction wave. Subsequent uterine contractions arise with the appearance of a new portion of acetylcholine. If The Mechanism of timely rhythmic breakdown of acetylcholine is disrupted, the delayed presence of the latter in contact with ganglion cells rapidly shifts the excitatory effect of the neuromuscular apparatus to a depressive effect—uterine contractions diminish or disappear entirely.

The sensitivity of uterine receptors depends on the hormonal Background, primarily on The ratio of sex Steroid Hormones: estrogens and progesterone. The lower the progesterone-to-estrogen index, the higher the body's readiness for labor.

In the regulation of uterine motor function, alongside Hormonal Influences, a certain role belongs to serotonin, acetylcholine, kinins, and mediator Enzymes through whose system hormones affect bioelectrical and plastic processes in the myometrium. As the quantities of the aforementioned factors increase, the contractile activity of the uterus is enhanced. There is an accumulation of energy substrates (Glycogen, phosphocreatine, Actomyosin) and electrolytes (calcium, magnesium, potassium, sodium). The uterus contains a group of cells where excitation arises first and then spreads further. This is known as the pacemaker, which is located in the uterine fundus near the right tubal angle.

An important regulator of uterine contractile activity is oxytocin, a hormone of the posterior pituitary and Hypothalamus. Many authors view it as the triggering factor in The Development of labor. The mechanism of oxytocin's action on the myometrium is associated with a decrease in Muscle cell Membrane Potential. It is well established that The transport of sodium and Calcium Ions plays a certain role in this process. Oxytocin affects The rate of acetylcholine binding by myometrial receptors and participates in the mechanism of acetylcholine release from its bound state. The effect of oxytocin is manifested through the inhibition of cholinesterase activity, alterations in cellular electrolyte properties, and the stimulation of uterine a-adrenoceptors. Consequently, as the level of this hormone increases prior to labor, uterine contractility is enhanced. Oxytocin's action is most pronounced during the progression of labor, aiming to conclude it and prevent postpartum Hemorrhage.

Currently, it is believed that the primary role in the development of labor activity is played by Prostaglandins (E₂, F₂a), the synthesis of which in the decidual and amniotic membranes increases significantly prior to labor.

Prostaglandin Biosynthesis is activated by steroid hormones, in which the fetoplacental unit plays a major role. An increase in estrogen levels produced by the mother-Placenta-fetus system leads to a rise in uterine prostaglandin content. The Role of adrenal cortisone in this process is also not excluded. Prostaglandins induce labor, cause depolarization of the myometrial cell membrane, and facilitate the release of bound Ca²⁺, thereby triggering uterine muscle contractions.

Histamine—a biogenic amine that exerts a vasodilatory effect on the smooth Muscles of the uterus—also holds some significance in the development of labor activity. It facilitates The production of oxytocic substances by the Pituitary Gland.

Vitamins also play a role in the initiation and progression of labor. Some of them act as biogenic stimulators of uterine contractility.

Functional disorders in the nervous system, ranging from the Cerebral Cortex down to the receptor apparatus of the uterus inclusive, can lead to the development of uterine contractile dysfunctions. In women During the first trimester of pregnancy (up to 12 weeks), There is a sharp increase in the excitability of the cerebral cortex, after which it decreases significantly and remains stable until 38 weeks of gestation. Two weeks before labor, Brain excitability drops sharply and remains at that level until the onset of labor. The sharp decline in brain excitability toward the end of pregnancy is accompanied by an increase in Spinal Cord excitability and, consequently, an increase in A number of spinal reflexes related to the development of labor activity.

Labor occurs in the presence of an established labor dominant, which integrates both higher neural centers and effector Organs into a dynamic system. METABOLISM/18.html">The Influence of Sex Hormones on various structures of the central and peripheral nervous systems is of great importance in The formation of the labor dominant. The central nervous system exerts higher-level regulation over the labor process. A significant increase in the electrical activity of the brain is noted 1.5 to 1 weeks before the onset of labor.

A crucial factor in the onset and proper progression of labor is the preparedness of the female body, the readiness of the uterus, and the sensitivity of the myometrium to contractile substances.

The specific course of labor activity depends largely on the degree of the pregnant woman's body readiness for childbirth. The formation of this readiness takes place during the final 10–15 days of gestation preceding labor. It is determined by the degree of cervical maturity and the sensitivity of the myometrium to oxytocic agents. Various methodologies exist for assessing cervical "maturity".

For clinical practice, it is advisable to use a methodology that accounts for the maximum possible number of state variants while remaining sufficiently simple to apply. Based on these principles, A.P. Golubev developed a scoring scale for cervical "maturity" in 1972. The scale incorporates such indicators of cervical maturity as the consistency of the vaginal portion of the cervix, its length, and the patency of the cervical canal. Each of these parameters is assigned a score from zero to 4 depending on its degree of expression. First, each parameter is evaluated individually, and then the scores of all parameters are summed up. The resulting sum serves as the overall indicator of the degree of cervical maturity. According to this scale, the state of the cervix can be rated from zero to 12 points. Thus, using this scale, 13 Variants of the cervical state can be distinguished. With a total score of 0 to 4 points, the cervix is considered immature; 5 to 8 points indicate a ripening cervix; and 9 to 12 points signify a mature cervix prepared for labor.

Class="center">Cervical Maturity Scoring Scale (A.P. Golubev, 1972)

Total score

With a total score of 0-4, the cervix is immature.

With a total score of 5-8, the cervix is ripening.

With a total score of 9-12, the cervix is mature.

The Classification of cervical maturity developed by Bishop (1964) is widely used internationally.

Bishop scoring system for cervical maturity (Bishop, 1964).

If the total score exceeds 10, the cervix is considered ripe (favorable for labor), and induction of labor can be performed.