Labor Physiology

Blood Supply

A major portion of the blood supply to the pelvis is via the branches of the internal iliac artery. The vascular supply of the uterus is derived principally from the uterine and ovarian arteries. The uterine artery, a main branch of the internal iliac artery, enters the base of the broad ligament, and makes its way medially to the side of the uterus.

Mechanisms of Dilatation

Compared with the body of the uterus, the lower uterine segment and cervix are regions of lesser resistance. Therefore, during a contraction, these structures are subjected to distention, in the course of which a centrifugal pull is exerted on the cervix. As the uterine contractions cause pressure on the membranes, the hydrostatic action of the amniotic sac, in turn, dilates the cervical canal like a wedge. In the absence of intact membranes, the pressure of the presenting part against the cervix and lower uterine segment is similarly effective. Early rupture of the membranes does not retard cervical dilatation so long as the presenting part of the fetus is positioned to exert pressure against the cervix and lower uterine segment.

Mechanisms of Uterine Contractions

Coordination is mediated by the low resistance gap junctions between myometrial cells that promote propagation of the action potential throughout the uterus.

Mechanisms of Cervical Effacement

Effacement of the cervix is shortening of the cervical canal from a length of about 2 cm so that the canal is replaced by a mere circular orifice with almost paper-thin edges. It occurs as the muscular fibers in the vicinity of the internal os are pulled upward, or "taken up", into the lower uterine segment, while the condition of the external os remains temporarily unchanged. The edges of the internal os are drawn upward several centimeters to become a functional part of the lower uterine segment.

Expulsion of the placenta

Expulsion of the placenta occurs after placental separation. If the patient is awake, she is asked to bear down while gentle traction is made on the umbilical cord. If the patient is asleep or unable to bear down, pressure is made on the uterine fundus and the placenta expressed.

Innervation

Nerve supply is derived principally from the sympathetic nervous system, but also partly from the cerebrospinal and parasympathetic systems. The 11th and 12th thoracic nerve roots are the sensory fibers from the uterus that transmit painful stimuli of uterine contractions to the central nervous system. The sensory nerves from the cervix and upper part of the vagina pass through the pelvic nerves to the 2nd, 3rd, 4th sacral nerves, whereas those from the lower portion of the vagina pass primarily through the pudendal nerve

Signs suggesting that detachment has taken place include:

1) trickle or gush of blood from the vagina 2) Lengthening of the umbilical cord outside the vulva 3) Rising of the uterine fundus in the abdomen as the placenta passes from the uterus into the vagina 4) Uterus becoming firm and globular

Estrogen

Begin to rise at 34-35 weeks, and reach a plateau at about 38 weeks. Estrogen promotes formation of gap junctions, increases oxytocin and estrogen receptors in the myometrium, enhances lipase activity and release of arachidonic acid, thus stimulating prostaglandin production, increases binding of intracellular calcium and increases myosin phosphorylation - has no direct effect on contractility but exerts a regulatory influence. Estrogen production by the placenta comes from fetal adrenal precursors and fetal factors may also play a critical role in the onset of labor. The amniotic concentrations of estradiol and estrone increase 15-20 days prior to the onset of term or preterm labor.

Oxytocin

Endogenous oxytocin is synthesized in the neural cell bodies in the hypothalamus and transported to nerve terminals in the posterior pituitary. Release of oxytocin into the bloodstream is stimulated by suckling, genital stimulation, and stretching of the cervix. The binding of oxytocin to receptors on the cell membrane increases the frequency of pacemaker potentials and lowers the threshold for initiation of action potentials. Under the influence of estrogen, the sensitivity of the myometrium to the effects of oxytocin changes markedly during pregnancy, so that oxytocin does not work well as a labor stimulant for induction of labor prior to term pregnancy.

Latent phase of the first stage of labor

Includes the period of time from the beginning of true regular contraction to the point when cervical dilation begins to progress rapidly. Generally, this is from onset of true contractions to 3-6cm dilation or the beginning of the active phase of labor. Little to no descent of the presenting part occurs during this phase. Contractions become coordinated, stronger, polarized, and more efficient. Contractions in this phase occur every 10 to 20 minutes and last about 15 to 20 seconds with mild to moderate intensity and increase to a frequency of every 5-7 min lasting 30-40 sec. At the same time the cervix is becoming softer, more pliable, and more elastic.

Progesterone

Pharmacological inhibition of progesterone action increases myometrial contractility, and that it is likely that there is actually no functional withdrawal of progesterone, but its 'pro-pregnancy' activity is overwhelmed by 'pro-labour' factors. Progesterone is thought to suppress uterine excitement throughout gestation. Labor is not preceded by a significant fall in maternal serum progesterone, rather, changes within fetal membranes and decidua may be triggered by localized increases in estrogen synthesis and decreases in progesterone formation. The decreased availability of progesterone to the myometrial cells allows estrogen effects to dominate.

The placenta is expelled by one of two mechanisms:

Schultz: the delivery of the placenta with fetal side presenting. This is the most common mechanism. (Shiny Shultz) Duncan: the delivery of the placenta with maternal side presenting. (Dirty Duncan)

Anal sphincter

The external anal sphincter consists of two strata of fibers (superficial and deep), which together form one flat plane of muscular fibers; it arises from the anococcygeal body, which is a tendinous band extending from the tip of the coccyx to the posterior margin of the anus; it passes around, encircles, and surrounds the anal canal; it inserts in the central tendinous point of the perineum.

First Stage

The first stage of labor begins with true labor contractions that result in progressive cervical dilation and effacement and ends with the cervix dilated 10 cm. The first stage of labor is comprised of a latent, active, and transition phase.

Fourth Stage (Recovery and Bonding)

The fourth stage lasts about 1 to 4 hours. It begins with the birth of the placenta until the postpartum condition of the patient has become stabilized. A uterus that is located centrally above the umbilicus suggests the presence of blood and clots, which should be expressed. A uterus above the umbilicus and displaced (usually to the right) is indicative of a full bladder.

Perineal Floor Musculature: Levator Ani.

The levator ani comprises the largest portion of the pelvic floor and supports the abdominal and pelvic organs; a broad muscular sling that originates from the posterior surface of the superior rami of the pubis, from the inner surface of the ischial spine, and between these two sites, from the obturator fascia. The muscle fibers are inserted in several locations as follows: around the vagina and rectum to form efficient functional sphincters for each; into a raphe in the midline between the vagina and rectum; into a midline raphe below the rectum; and into the coccyx.

Prostaglandins

The major sources of prostaglandins involved in labor onset are the fetal membranes, decidua, placenta, and uterus. Stretch of the uterus and cervix, estrogens (especially estradiol), and oxytocin also stimulate synthesis of prostaglandins. It is postulated that the critical point in the onset of labor is stimulation of PGE2 synthesis in the amnion. PGE2 is then transferred across the chorion and amniotic fluid to the decidua. In the decidua PGE2 is converted to or acts as a stimulus for production of PGF2oc. Prostaglandins seem to have similar effects on the myometrium at all stages of pregnancy and thus can be used to initiate labor prior to term. Prostaglandins bind to the cell membrane, increase the frequency of action potentials, and stimulate actual muscle contraction.

Second Stage (Expulsive Stage)

The second stage of labor is known as beginning with complete dilation of the cervix (10 cm) and ending when the baby is born. As pressure on the cervix increases, women may have the sensation of pelvic pressure and an urge to begin pushing. Contractions in the second stage are considered severe and occur at 2-3 minute intervals with duration of 50 to 90 seconds, and become expulsive in nature. There are clinical indications that the second stage has started: 1) There is an increase in bloody show 2) The patient wants to bear down with each contraction 3) Feelings of pressure on the rectum accompanied by the desire to defecate 4) Rectal and perineal bulging and flattening 5) Nausea and retching occur frequently as the cervix reaches full dilatation. There are three phases of the second stage of labor: Phase I, the lull: Between the first and second stage of labor there is often a quiet or lull period, in which the ctx space out and are not as intense. The woman may even rest or nap. This period may last as long as an hour. Phase II, active bearing down: occurs from the onset of rhythmic bearing down or the urge to push until crowning Phase III, perineal: from crowning until birth of the entire body.

Third Stage (Placental Stage)

The third stage is marked from the birth of the baby to delivery of the placenta. It takes 5 to 30 minutes. There is a risk of hemorrhage if the third stage last longer than 30 min. It occurs in two phases: (1) separation of the placenta from the wall of the uterus and into the lower uterine segment and/or vagina (2) actual expulsion of the placenta out of the birth canal. Both separation and expulsion are caused by uterine contractions. Ctx during this stage occur every 4-5 min until the placenta is expelled.

Transition phase of the first stage of labor

The transition from long-lasting, irregular contractions of low frequency to high -intensity, high frequency, regularly spaced contraction is associated with the progressive dilatation and effacement of cervix and there is an irresistible urge to push typically occurs when the fetal presenting part reaches +1 station. Women may also experience some or all of the following S+S: perspiration, shaking, teeth chattering, leg and buttock cramps, hiccupping, belching, thirst, anorexia, anxiety, N+V, restlessness, increased tenderness to touch over abdomen and back, amnesia between ctx, difficulty comprehending instructions, ctx q 1.5-2 mins lasting 60-90 sec of severe intensity seeming almost constant and quite painful, generalized discomfort, bewilderment, frustration, severe low back pain, decrease in sense of modesty, irritability, pulling or stretching sensation in pelvis, apprehension, increase in bloody show, rectal pressure, ROM, rectal and perineal bulging, and expulsive grunt upon exhalation

Bulbocavernosus

There are two bulbocavernosus muscles, one on either side of the vaginal orifice; posteriorly they attach to the central tendinous point of the perineum and the inferior fascia of the urogenital diaphragm; anteriorly they insert into the corpora cavernosa clitoridis; laterally they surround the orifice of the vagina, covering the vestibular bulbs and Bartholin glands on either side. Their function is to contract and reduce the size of the vaginal orifice; the anterior muscle fibers contribute to clitoral erection.