Obstetrics and Gynecology - A. M. Gromova 2000

Physiology of Labor
Clinical Course of Labor

Labor is divided into three stages: the first is the cervical dilation stage, the second is the fetal expulsion stage, and the third is the placental stage.

The First stage of labor begins with regular contractions occurring every 10-15 minutes, which gradually become more frequent and intense, leading to structural Changes in the cervix (effacement and dilation of the internal os), and concludes with full dilation (10-12 cm) of the external os. At this degree of dilation, the cervix allows the HEAD and body of a mature fetus to pass through.

The zone of spontaneous uterine excitation (the "pacemaker") arises at the onset of labor in the uterine cornu (most commonly the right) and spreads from the fundus to the body, gradually fading in the lower uterine segment, which clinically manifests as its relaxation and stretching. As labor progresses, the locking function of the cervix gradually disappears due to a decrease in Blood levels of progesterone and relaxin (endogenous Hormones produced by the Placenta).

Cervical effacement in primigravidae occurs sequentially, beginning at the external os and gradually involving the entire cervix, whereas in multiparous women it occurs simultaneously, owing to some pre-existing dilation of the external os at the beginning of the first stage of labor.

The rate of cervical dilation at the beginning of labor (latent phase) is 0.35 cm/h; in the active phase (dilation from 3 to 8 cm), it is 1.5-2 cm/h in primigravidae and 2-2.5 cm/h in multiparous women. Dilation of the uterine os from 8 to 10 cm (deceleration phase) proceeds at a slower pace of 1-1.5 cm/h.

The rate of cervical dilation increases with The Use of antispasmodics and analgesics.

As a rule, by the end of the first stage, rupture of the amniotic sac occurs due to increased intrauterine pressure. As the fetal head is pressed against the pelvic inlet along its largest circumference, it is closely embraced by the soft Tissues of the birth canal, creating a "zone of contact". Consequently, the Amniotic Fluid is divided into forewaters and hindwaters. Following the rupture of the amniotic sac, the forewaters (100-150 ml) are discharged. The hindwaters (the main volume of amniotic fluid) are expelled immediately after the birth of the child.

The amniotic sac plays a vital role in the physiology of labor. It exerts pressure on the inner surface of the cervix, which contains A large number of receptors; their stimulation not only enhances spontaneous uterine contractions but also promotes cervical dilation. Sometimes the amniotic sac ruptures when dilation is incomplete, and occasionally even before the onset of labor. If it ruptures before full dilation of the os, it is referred to as early rupture of membranes; amniotic fluid discharge prior to the onset of labor is called premature rupture. Both early and Premature Rupture of membranes adversely affect the course of labor.

The course of labor comprises the latent phase, the active phase, and the deceleration phase.

The latent phase is the time interval from the onset of regular contractions to the appearance of structural changes in the cervix (until the uterine os is dilated to 3-4 cm). During the latent phase, contractile activity responds well to pharmacological intervention (tocolysis). The duration of the latent phase is 6-7 hours in primigravidae and 4-5 hours in multiparous women, depending on the state of cervical ripening and obstetric history, as well as pharmacological influence, while being independent of fetal weight. Occasionally, the latent phase cannot be clinically detected, as cervical effacement and dilation up to 3-4 cm occur during the 4 weeks preceding labor.

The latent phase is followed by the active phase of labor, which is characterized by rapid dilation of the uterine os. This phase comprises initial acceleration, maximal slope, and deceleration. The active phase of labor occurs when cervical dilation reaches 3-8 cm.

The deceleration phase is attributed to the cervix slipping over the fetal head at the end of the first stage of labor, coinciding with the rapid descent of the head. The duration of the dilation stage is 8-10 hours in nulliparous pregnant women and 6-8 hours in multiparous women.

The Second Stage of labor (the expulsion stage) begins after full cervical dilation (10-12 cm). Uterine contractions are joined by maternal pushing efforts (bearing-down efforts). These efforts occur reflexively as the presenting part of the fetus stimulates nerve elements located in the cervix, parametrial tissue, and pelvic floor Muscles. Although involuntary, the parturient woman can regulate them to a certain extent (enhance or inhibit them). They play a role only during the expulsion of the fetus and placenta. Intra-abdominal pressure increases during bearing-down efforts. The simultaneous increase in intrauterine pressure (contractions) and intra-abdominal pressure (pushing) directs the uterine contents toward the path of least resistance, namely the lesser pelvis.

During the second stage of labor, the biomechanism of labor is completed. The average duration of the second stage is 1.5 hours in first-time deliveries and 30-60 minutes in subsequent deliveries. Initially, the fetal head appears at the vaginal opening only during pushing efforts (crowning of the head), after which the head advances and no longer recedes between contractions (birth of the head).

The occipital region of the fetal head is born first, followed by the parietal eminences emerging from the vaginal opening. After the birth of the occiput and vertex, the fetal forehead and face appear from the birth canal, at which point perineal tension reaches its maximum. With continued pushing efforts, the fetal trunk rotates, causing the head to undergo external rotation. The birth of the shoulders, torso, and lower limbs completes the delivery of the fetus.

The Management of Labor consists of obstetric assistance to the parturient woman, or "perineal protection," which is a set of successive manipulations at the end of the second stage of labor aimed at facilitating the physiological mechanism of labor and preventing birth trauma.

The Third Stage of labor is the placental stage. It begins immediately after the birth of the infant. Driven by uterine contractions, the placenta detaches from the uterine wall, while the expulsion of the products of conception from the birth canal is accomplished through uterine contractions and maternal pushing efforts. The rate of placental Separation and expulsion from the Uterus is directly dependent on the intensity of uterine motor function. Two Phases of the placental stage are distinguished:

1. The phase of placental separation.

2. The phase of placental expulsion (delivery of the afterbirth).

In The first phase, placental separation may begin centrally, forming a retroplacental hematoma (Schultze mechanism). Since blood cannot escape *vis a tergo*, during the next contraction it promotes complete detachment because the center of the placenta bulges into the uterine cavity and retraction progressively involves an increasing number of Muscle bundles in an eccentric manner. With this mechanism of placental separation, the first phase is bloodless, and the afterbirth is delivered with the placenta inverted so that the fetal (outer) and maternal (inner) membranes enclose the blood accumulated during separation.

When placental separation begins at the periphery (Duncan mechanism), moderate venous blood loss occurs During the first phase of the placental stage, and the placenta is delivered edge-first with inverted membranes, without blood trapped between them.

The phase of placental expulsion proceeds under the action of pelvic floor muscle contractions and reflex or spontaneous bearing-down efforts, sometimes independently of uterine contractions.

A single pushing effort, occurring painlessly, is sufficient for the delivery of the afterbirth. Clinical management of the placental stage involves expectant-active care. It is aimed at the timely recognition of placental separation from the uterine walls. The latter is diagnosed based on a series of clinical signs (Schroeder, Ahlfeld, Mikulicz, Chukanov-Küstner, etc.). Rational maneuvers that facilitate the delivery of the afterbirth are employed, such as the Abuladze, Ganter, and Credé-Lazareвич Methods.

The placental stage is always accompanied by blood loss. Physiological blood loss is considered to be up to 250 ml, or up to 0.5% of the parturient woman's body weight. Pathological blood loss exceeds 0.5% of body weight. The duration of the placental stage ranges from 5 to 30 minutes.