Labor (parturition) refers to the series of events that expel the fetus out of the mother's body by means of uterine contractions. The musculature of the pregnant uterus is arranged in three strata: an external hood-like layer, which arches over the fundus and extends into the various ligaments; an internal layer consisting of sphincter-like fibers around the orifices of the tubes and the internal Os; and the main portion of the uterine wall, formed by a middle layer, which consists of an interlacing network of muscle fibers. The uterus has pacemakers to produce the rhythmic coordinated contractions of labor.
Parturition results from a combination of several different factors that progressively increase the excitability of the uterine musculature. The placenta at term begins to secrete a higher ratio of estrogen (which excites uterine activity) to progesterone (which inhibits uterine activity). The fetal size produces a stretching of uterine musculature that also increases its excitability. The fetal presenting part (usually the head) mechanically presses downward on the maternal pelvis, stretching the cervix, which in turn, increases uterine activity. Relaxin, produced by the ovary, increases cervical dilation and relaxation of pelvic ligaments. Its levels increase just prior to parturition. The posterior pituitary gland increases secretion of the hormone oxytocin, while at the same time, uterine muscle cells increase the number of oxytocin receptors, increasing the uterus' sensitivity to oxytocin. Thus, the fetal presenting part stretches the cervix, which causes a greater increase in uterine activity, due to both mechanical pressure and biochemical action (signals from the cervix to the hypothalamus, causing an increased oxytocin production). As a result of these factors, the rhythmic contractions of the uterus become strong enough to begin pushing the fetus into the birth canal.
Normal labor consists of three stages. The first stage is the dilation stage, from the onset of labor to complete dilation of the cervix. The cervix dilates up to 10 cm in diameter. The first stage is divided in two substages, the latent phase that encompasses cervical effacement (thinning) and early dilatation. The second is the active phase, during which more rapid cervical dilatation occurs (starting from 3 to 4 cm). During the dilation stage, the amniotic fluid is expelled ("water breaks").
The second tage of labor is the expulsion stage, and encompasses complete cervical dilation throughout the delivery of the infant. The placental stage (stage three) begins after the delivery of the infant and ends with the expulsion of the placenta. Stage three occurs within 5-30 minutes after delivery of the infant.
The mechanism of labor consists in several changes of position (cardinal movements) of the fetus as it passes through the birth canal. This accommodation is accomplished by the forceful contractions of the uterus. In other words, the fetal head adapts to the maternal bony pelvis, and the lower most presenting part of fetus (usually the occipital portion of the head) rotates toward the largest pelvic segments. The headfirst, or vertex, presentation of the infant occurs in 95% of term labors. The cardinal movements include engagement, the descent of biparietal diameter of fetal head below the pelvic inlet, descent of the presenting part through the birth canal (mostly occurring during the latter portion of the first stage and the second stage of labor), flexion of the fetal head to present the smallest diameter of the fetal head to the maternal pelvis, internal rotation that facilitates the presentation of the optimal diameters of the fetal head to the bony pelvis, extension of the fetal head as it reach the vaginal introitus, external rotation of the fetal head after delivery to "face forward" relative to his shoulders, and finally, expulsion, the final delivery of the fetus from the birth canal.
This is the complete article, containing 612 words
(approx. 2 pages at 300 words per page).
