PHYSIOLOGY OF LABOR IN ENGLISH

                                                     

                                  PHYSIOLOGY OF LABOR IN ENGLISH

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Physiology of Labor-

Physiology of labor is a Series of events that take place in the genital organs in an effort to expel the viable products of  conception  into the outer world. It includes events in first stage of labor, events in second stage of labor and events in third stage of labor.

Events in first stage of LABOR-

Physiological events in first stage of labor includes-

(1) Contractions of uterus- there are wide variations in frequency, intensity and duration of contractions, having following patterns.

Ø  There is good synchronization of the contraction waves from both halves of the uterus and also between upper and lower uterine segments.

Ø  There is fundal dominance of contractions that diminish gradually in duration and intensity

Ø  through midzone down to lower segment. It takes about 10–20 seconds.

Ø  The waves of contraction follow a regular pattern.

Ø  The upper segment of the uterus contracts more strongly and for a longer time than the lower 

Ø  Intra-amniotic pressure rises beyond 20 mm Hg during uterine contraction.

Ø  Good relaxation occurs in between contractions to bring down the intra-amniotic pressure to less than 8 mm Hg. Contractions of the fundus last longer than that of the midzone.

During uterine contractions the patient experiences pain which is situated more on the hypogastric region, often radiating to the thighs.  causes of pain are:

(a)    Myometrial hypoxia during contractions 

(b)    stretching of the peritoneum over the fundus,

(c)    stretching of the cervix during dilatation,

(d)    stretching of the ligaments surrounding the

uterus and (e) compression of the nerve ganglion.

Tonus : It is the intrauterine pressure in between contractions. During pregnancy, as the uterus is inactive, the tonus is of 2–3 mm Hg. During the first stage of labor, it varies from 8 mm Hg to 10 mm Hg. The factors which affects the tonus are:

(i) Contractility of uterine muscles,

(ii)intra-abdominal pressure, and

(iii) overdistension of uterus as in twins and hydramnios.

Intensity : The intensity of uterine contraction describes the degree of uterine systole. The intensity gradually increases with advancement of labor until it becomes maximum in the second stage during delivery of the baby. Intrauterine pressure is raised to 40–50 mm Hg during first stage and about 100–120 mm Hg in second stage of labor during contractions. 

Duration : In the first stage, the contractions last for about 30 seconds initially but gradually increase in duration with the progress of labor. Thus in the second stage, the contractions last longer than in the first stage.

Frequency: In the early stage of labor, the contractions come at intervals of 10–15 minutes. The intervals gradually shorten with advancement of labor until in the second stage, when it comes every 2–3 minutes.

(2) Retractions of uterus- Retraction is a phenomenon

of the uterus in labor in which the muscle fibers are permanently shortened. the uterine muscles have this property to become shortened once and for all.

Effects of retraction-  — Essential property in the formation of lower uterine segment and dilatation and effacement of the cervix. — To maintain the descent of the presenting part made by the uterine contractions and to help in ultimate expulsion of the fetus. — To reduce the surface area of the uterus favoring  separation of placenta. — Effective hemostasis after the separation of the placenta.

(3) Dilatation of the cervix-   Dilatation of cervix starts at onset of labor. In multipara it may start a little earlier.

Predisposing factors which favor smooth dilatation are—

(a) softening of the cervix,

(b) fibromusculoglandular hypertrophy,

(c) increased vascularity, 

(d) accumulation of fluid in between collagen fibers,

(e) breaking down of collagen fibrils by enzymes collagenase and elastase, and

(f) Increase in hyaluronic acid, decrease in dermatan sulfate in the matrix of the cervix. These are under the action of hormones—estrogen, progesterone and relaxin

Actual factors which favor smooth cervical dilatation are—

Uterine contraction and retraction-  Each uterine contraction, not only the cervical canal is opened up from above down but also it becomes shortened and retracted.

Fetal axis pressure: In labor with longitudinal lie and with well-fitted (flexed) fetal head on the cervix, fetal vertebral column is straightened by the contractions of the uterus. This allows the fundal strong contraction force to be transmitted through the fetal podalic pole and vertebral column to the well-fitted fetal head. This causes mechanical stretching of the lower segment and dilatation of the cervical canal.

Bag of membranes:  During uterine contractions the bag of membranes or bag of waters (forewater)  generate hydrostatic pressure that in turn dilate the cervical canal .

EFFACEMENT OF CERVIX:

Effacement is the process by which the muscular fibers of the cervix are pulled upward and merges with the fibers of the lower uterine segment. The cervix becomes thin during first stage of labor or even before that in primigravidae

LOWER UTERINE SEGMENT: Before the onset of labor, there is no complete anatomical or functional division of the uterus. During labor the demarcation of an active upper segment and a relatively passive lower segment is appeared. This demarcation is known as physiological retraction ring

Events in second stage of LABOR-

The second stage begins with the complete  dilatation of the cervix and ends with the expulsion of the fetus. This stage is concerned with the descent and delivery of the fetus through the birth canal. Second stage has two phases:

(1) Propulsive—from full dilatation until head touches the pelvic floor.

(2) Expulsive—since the time mother has irresistible desire to “bear down” and push until the baby is delivered.

In second stage cervix is fully dilated and  the membranes usually rupture and there is escape of good amount of liquor amnii. The volume of the uterine cavity is thereby reduced. Simultaneously, uterine contraction and retraction become stronger. The uterus becomes elongated during contraction. The elongation is partly due to the contractions of the circular muscle fibers of the uterus to keep the fetal axis straight.

Delivery of the fetus is accomplished by bearing down effort that is the downward thrust  provided by uterine contractions supplemented by voluntary contraction of abdominal muscles against the resistance offered by bony and soft tissues of the birth canal.

 

Events in third stage of LABOR-

The third stage of labor is the phase of placental separation; its descent to the lower segment and finally its expulsion with the membranes.

Mechanism of separation: Marked retraction reduces the surface area at the placental site to about its half. But as the placenta is inelastic, it cannot keep pace with such an extent of diminution resulting in its buckling

shearing force is instituted between the placenta and the placental site which brings about its ultimate separation. The plane of separation runs through deep spongy layer of decidua basalis so that a variable thickness of decidua covers the maternal surface of the separated placenta. There are two ways of separation of placenta (1) Central separation  and (2) Marginal separation

(1) Central separation – Detachment of placenta from its uterine attachment starts at the center resulting in opening up of few uterine sinuses and accumulation of blood behind the Placenta. With increasing contraction, more and more detachment occurs facilitated by weight of the placenta and retroplacental blood until whole of the placenta gets detached.

(2) Marginal separation - Separation starts at the margin as it is mostly unsupported. With progressive uterine contraction, more and more areas of the placenta get separated. Marginal separation is found more frequently.

SEPARATION OF THE MEMBRANES: The membranes, which are attached loosely in the active part, are thrown into multiple folds. Those attached to the lower segment are already separated during its stretching.

EXPULSION OF PLACENTA: After complete separation of the placenta, it is forced down into the

flabby lower uterine segment or upper part of the vagina by effective contraction and retraction of the

uterus. Thereafter, it is expelled out either by voluntary contraction of abdominal muscles (bearing down efforts) or by manual procedure