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- Labor and birth are affected by the five Ps: passenger, passageway, powers, position of the woman, and psychologic response.
- Because of its size and relative rigidity, the fetal head is a major factor in determining the course of birth.
- The diameters at the plane of the pelvic inlet, the midpelvis, and the outlet plus the axis of the birth canal determine whether vaginal birth is possible and the manner in which the fetus passes down the birth canal.
- Involuntary uterine contractions act to expel the fetus and placenta during the first stage of labor; these are augmented by voluntary bearing-down efforts during the second stage.
- The first stage of labor lasts from the time dilation begins to the time when the cervix is fully dilated.
- The second stage of labor lasts from the time of full cervical dilation to the birth of the infant.
- The third stage of labor lasts from the infant's birth to the expulsion of the placenta.
- The fourth stage of labor begins with the delivery of the placenta and includes at least the first 2 hours after birth.
- The cardinal movements of the mechanism of labor are engagement, descent, flexion, internal rotation, extension, restitution and external rotation, and expulsion of the infant.
- Although the events precipitating the onset of labor are unknown, many factors, including changes in the maternal uterus, cervix, and pituitary gland, are thought to be involved.
- A healthy fetus with an adequate uterofetoplacental circulation is able to compensate for the stress of uterine contractions.
- As the woman progresses through labor, various body systems adapt to the birth process.
The cervix and vagina are soft tissues that form the part of the passageway known as the birth canal. In early pregnancy the cervix is long, firm, and closed. As the time for delivery approaches, the cervix usually begins to soften.
Then when labor begins uterine contractions affect the cervix in two ways. First, the cervix begins to get shorter and thinner, a process called effacement. Cervical effacement is recorded as a percentage. The cervical canal measures approximately 2 cm before effacement. At a length of 1 cm, the cervix is 50% effaced. When the cervix is paper thin, it is 100% effaced.
The second cervical change that occurs during normal labor is dilation. The cervix must open to allow the fetus to be born. Dilation is measured in centimeters and is considered completely dilated at 10 cm.
Normally a primiparous woman experiences effacement before dilation while for a multiparous woman, both processes usually occur at the same time. Often the multipara's cervix dilates 1 to 2 cm several weeks before labor begins.
The vaginal canal participates in childbirth via passive distention. During birth, the rugae of the vaginal walls stretch and smooth out, allowing for considerable expansion. The muscles and soft tissues of the primipara provide greater resistance to stretching and distending than those of the multipara. This is one reason why the first baby often takes longer to be born than subsequent babies.
The primary force of labor comes from involuntary muscular contractions of the uterus. These labor contractions cause effacement and dilation of the cervix during the first stage of labor.
Secondary powers are voluntary muscle contractions of the maternal abdomen during the second stage of labor that help expel the fetus.
Each involuntary uterine contraction is composed of three phases: increment, acme, and decrement, followed by a relaxation period. The increment or building up of the contraction is the longest phase. During the increment, the contraction gains strength until is reaches the acme, or peak, of the contraction. The decrement is the letting-up phase, as the contraction relaxes gradually to baseline.
Document contractions using three descriptors: frequency, duration, and intensity. Frequency refers to how often the contractions are occurring and is measured by counting the time interval from the beginning of one contraction to the beginning of the following contraction.
Duration is the interval from the beginning of a contraction to its end. Intensity refers to the strength of the contraction. Estimate intensity by palpating the fundus at the peak of the contraction and record it as mild, moderate or strong. Intensity can be measured directly with an intrauterine pressure transducer.
Each contraction constricts the blood vessels that supply the placenta, thereby decreasing the amount of oxygen that flows to the fetus. The relaxation period allows the vessels to fill with oxygen-rich blood to supply the uterus and placenta. Relaxation is also necessary so that maternal muscles do not become overly fatigued and to allow the laboring woman momentary relief from the pain of labor.
Current Pregnancy experience - Unplanned versus planned pregnancy, amount of difficulty conceiving, presence of risk factors or complications of pregnancy
Previous birth experiences - Positive or negative feelings regarding previous delivery experience, complications encountered during previous delivery, mode of delivery (vaginal versus cesarean), birth outcomes (e.g., fetal demise, birth defects)
Expectations for current birth experience - View of labor as a meaningful or stressful event, realistic and attainable goals versus idealistic views that conflict with reality (a situation that can lead to disappointment)
Preparation for birth - Type of childbirth preparation, familiarity with institution and its policies and procedures, type of relaxation techniques learned and practiced
Presence and support of a birth companion
Culture - A woman's culture influences and defines:
The childbirth experience - shameful versus joyful, superstitions and beliefs about pregnancy and birth, prescribed behaviors and taboos during the intrapartum period.
Relationships - Interpersonal interactions, parent-infant interactions, role expectations of family members, support person involvement
Pain - Meaning and context of pain during labor, acceptable responses to pain during labor
The significance of touch - Soothing versus intruding, May be a symbol of intimacy.
Approximately 2 weeks before labor, the presenting part may settle into the pelvic cavity, causing the pregnant woman to sense that the baby has "dropped." This subjective feeling is called "lightening." The woman is able to breathe more easily and may need to urinate more frequently because of the pressure of the fetus on the urinary bladder.
Braxton Hicks contractions occur more frequently and are more noticeable as pregnancy approaches term. These irregular, practice contractions usually decrease in intensity while walking and position changes.
The woman may experience gastrointestinal disturbances, such as diarrhea, heartburn, or nausea and vomiting as labor approaches.
Sometimes the mucus plug is expelled a week or two before labor begins. When the mucus plug passes, the woman will notice a one time clear or pink-tinged discharge that is the consistency of jelly.
She may have a burst of energy 24-48 hours before the onset of labor and may also have the desire to do heavy cleaning or some other big project in anticipation for the baby's arrival, a phenomenon known as the nesting urge. Caution the woman regarding the nesting urge, and advise her to conserve her energy for the work of labor.
Clinical signs that labor is approaching include ripening or softening and effacement (thinning) of the cervix.
Dilation of the cervix may accompany softening and effacement, particularly in multiparous women. The practitioner will inform the woman of these changes when a pelvic exam is done during the scheduled office visit.
The turns and movements made during the journey are referred to as the mechanisms of delivery or Cardinal movements. These include engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion. While they are discussed separately, it is important to remember that these mechanisms may overlap or occur simultaneously.
Engagement - Initial descent of the fetal head may result in engagement when the presenting part descends to the level of the ischial spines. Engagement can occur as early as two weeks before labor or not until after the onset of labor. Engagement is more likely to occur earlier in the primigravida and later in the multigravida.
Descent - Descent may begin before labor when the fetus "drops." Descent is measured by station, which is the relationship of the fetal-presenting part to the maternal ischial spines. Descent continues throughout labor to varying degrees.
Flexion - As the head descends during labor, the fetus encounters resistance from the soft tissues and muscles of the pelvic floor. This resistance normally coaxes the fetus to assume the attitude of flexion. Flexion is the attitude that presents the smallest diameters of the fetal head to the dimensions of the pelvis.
Internal Rotation - Frequently, in early labor, the fetal head presents to the pelvis in a transverse position because the inlet of the pelvis is widest from side to side. During active labor, the fetal head typically rotates 45 degrees from a transverse position to an anterior position so that the head can accommodate the pelvic outlet, which is wider from the front to back. This movement is called internal rotation. If the fetus does not rotate, the widest diameters of the fetal head present to the outlet of the pelvis, resulting in a less than optimal fit between the head and the bony passageway. This can prolong labor.
Extension - Typically, the fetal head is well flexed with the chin on the chest as the fetus travels through the birth canal. When the fetus reaches the pubic arch, it must extend under the symphysis pubis.
External Rotation - As the head is born, external rotation lines the head up with the shoulders.
Expulsion - Expulsion (birth) occurs after deliver of the anterior and posterior shoulders.
The first stage of labor begins with the onset of true labor and ends with full dilation of the cervix at 10 cm. This stage is divided into three phases: latent, active and transition.
Early Labor (Latent
Phase)
Begins when contractions of true labor start and ends when the cervix is dilated 4 cm. Contractions are usually mild intensity and typically occur at a frequency of five to ten minutes (although they can occur as infrequently as every 30 minutes) with a duration of 30 to 45 seconds.
In a normal labor, the pattern of contractions during the latent phase becomes increasingly regular with shorter intervals between contractions. The latent phase lasts on average approximately eight to nine hours for a primiparous woman but generally does not exceed 20 hours in length. Multiparous women usually experience shorter labors (an average length of five hours with an upper limit of 14 hours)
Active Labor (Active Phase)
The active phase begins at 4 cm cervical dilation and ends when the cervix is dilated 8 cm. Contractions typically occur every two to five minutes and last 45 to 60 seconds and are of moderate to strong intensity. Progressive cervical dilation and fetal
descent usually occur at this stage.
For Primiparas, dilation should occur at approximately 1.2cm/hr. Multiparas progress at a slightly faster rate of 1.5cm/hr. These designations are only approximations and may vary a great deal if the woman receives medication, anesthesia, or other medical intervention during labor. Fetal descent is often slow in the first stage of labor, regardless of parity. Occasionally, the fetus does not descend during active labor.
Transition (Transition
Phase)
Transition is the most difficult part of labor. This phase of the first stage of labor starts when the cervix is dilated 8 cm and ends with full cervical dilation. The contractions are strong of intensity, occur every two to three minutes, and are 60 to 90 seconds in duration.
Frequently the woman experiences a strong urge to push as the fetus descends. It is important for the woman to resist the urge to push until the cervix is dilated completely as pushing against a partially dilated cervix can cause swelling, which slows labor, or the cervix can develop lacerations, leading to hemorrhage.
There is increased demand for oxygen during the first stage of labor, attributable in part to the energy used for uterine contractions. to meet the demand there is a moderate increase in cardiac output throughout the first stage of labor. During the second stage, cardiac output may be increased as much as 40 to 50% above pre-labor levels. Immediately after birth, it may peach at 80% above the pre-labor level.
The pulse is often on the high end of normal during active labor. Dehydration and/or maternal exhaustion accentuate these normal increases in heart rate. Blood pressure however does not change appreciably during normal labor although the stress of contractions may cause a 15mm hg increase in the systolic pressure.
The increased demand for oxygen and the pain of the uterine contractions cause the respiratory rate to increase which puts the laboring woman at risk for hyperventilation. Mouth breathing and dehydration contribute to dry lips and mouth.
Labor prolongs the normal gastric emptying time. This change often leads to nausea and vomiting during active labor and increases the woman's risk for aspiration, particularly if general anesthesia is required. Traditionally the laboring woman receives IV fluids while solid food and fluids are withheld, but research shows that the risk for aspiration remains high even if the woman maintains an NPO status because gastric secretions become more acidic during periods of fasting. Current recommendations are to allow the laboring woman to have clear liquids, unless there is a high likelihood that she will deliver by cesarean.
Pressure on the urethra from the presenting part may cause overfilling of the bladder, a decreased sensation to void, and edema. A full bladder is uncontrollable and slows the progress of labor. As the bladder fills it rises upward in the pelvic cavity, which puts pressure on the lower uterine segment and prevents the head from descending. Sometimes a straight cath becomes necessary to empty the bladder.
Labor affects some lab values. The stress of vigorous labor may cause an increase in white blood cell count to as high as 30,000 cells/microliter (mcL). This increase is the body's normal response to inflammation, pain and stress. Frequently the urine specific gravity is high, indicating concentrated urine, and there may be a trace amount of urinary protein because of increased metabolic activity. Gross proteinuria is never normal during labor and is a sign of a developing complication.