Labor consists of a series of rhythmic, involuntary or medically induced contractions of the uterus that result in effacement (thinning and shortening) and dilation of the uterine cervix. The World Health Organization (WHO) defines normal birth as follows:
The birth is spontaneous in onset and low risk at the start of labor and remains so throughout labor and delivery.
The infant is born spontaneously in the vertex position between 37 and 42 weeks of pregnancy.
After birth, mother and infant are in good condition.
Benefits Of Exercise
The stimulus for labor is unknown, but digitally manipulating or mechanically stretching the cervix during examination enhances uterine contractile activity, most likely by stimulating release of oxytocin by the posterior pituitary gland.
Normal labor usually begins within 2 weeks (before or after) the estimated delivery date. In a first pregnancy, labor usually lasts 12 to 18 hours on average; subsequent labors are often shorter, averaging 6 to 8 hours.
Beginning of labor
Rupture of the chorioamniotic membranes or bloody show is diagnostic for onset of labor. Bloody show (a small amount of blood with mucous discharge from the cervix) may precede onset of labor by as much as 72 hours. Bloody show can be differentiated from abnormal 3rd-trimester vaginal bleeding because the amount is small, bloody show is typically mixed with mucus, and the pain due to abruptio placentae (premature separation) is absent. In most pregnant women, previous routine ultrasonography has been done and ruled out placenta previa. However, if ultrasonography has not ruled out placenta previa and vaginal bleeding occurs, placenta previa is assumed to be present until it is ruled out. In such cases, digital vaginal examination is contraindicated, and ultrasonography is done as soon as possible to determine the location of the placenta and rule out abruptio placentae. Labor begins with irregular uterine contractions of varying intensity; they apparently soften (ripen) the cervix, which begins to efface and dilate. As labor progresses, contractions increase in duration, intensity, and frequency.
Stages of labor
There are 3 stages of labor.
The 1st stage—from onset of labor to full dilation of the cervix (about 10 cm)—has 2 phases, latent and active.
During the latent phase, irregular contractions become progressively coordinated, discomfort is minimal, and the cervix effaces and dilates to 4 cm. The latent phase is difficult to time precisely, and duration varies, averaging 8 hours in nulliparas and 5 hours in multiparas; duration is considered abnormal if it lasts > 20 hours in nulliparas or > 12 hours in multiparas.
During the active phase, the cervix becomes fully dilated, and the presenting part descends well into the midpelvis. On average, the active phase lasts 5 to 7 hours in nulliparas and 2 to 4 hours in multiparas. Traditionally, the cervix was expected to dilate about 1.2 cm/hour in nulliparas and 1.5 cm/hour in multiparas. However, recent data suggest that slower progression of cervical dilation from 4 to 6 cm may be normal (1). Pelvic examinations are done every 2 to 3 hours to evaluate labor progress. Lack of progress in dilation and descent of the presenting part may indicate dystocia (fetopelvic disproportion).
Standing and walking shorten the first stage of labor by > 1 hour and reduce the rate of cesarean delivery (1).
If the membranes have not spontaneously ruptured, some clinicians use amniotomy (artificial rupture of membranes) routinely during the active phase. As a result, labor may progress more rapidly, and meconium-stained amniotic fluid may be detected earlier. Amniotomy during this stage may be necessary for specific indications, such as facilitating internal fetal monitoring to confirm fetal well-being. Amniotomy should be avoided in women with HIV infection or hepatitis B or C, so that the fetus is not exposed to these organisms.
During the 1st stage of labor, maternal heart rate and blood pressure and fetal heart rate should be checked continuously by electronic monitoring or intermittently by auscultation, usually with a portable Doppler ultrasound device (see fetal monitoring). Women may begin to feel the urge to bear down as the presenting part descends into the pelvis. However, they should be discouraged from bearing down until the cervix is fully dilated so that they do not tear the cervix or waste energy.
The 2nd stage is the time from full cervical dilation to delivery of the fetus. On average, it lasts 2 hours in nulliparas (median 50 minutes) and 1 hour in multiparas (median 20 minutes). It may last another hour or more if conduction (epidural) analgesia or intense opioid sedation is used. For spontaneous delivery, women must supplement uterine contractions by expulsively bearing down. In the 2nd stage, women should be attended constantly, and fetal heart sounds should be checked continuously or after every contraction. Contractions may be monitored by palpation or electronically.
During the 2nd stage of labor, perineal massage with lubricants and warm compresses may soften and stretch the perineum and thus reduce the rate of 3rd- and 4th-degree perineal tears (2). These techniques are widely used by midwives and birth attendants. Precautions should be taken to reduce risk of infection with perineal massage
During the 2nd stage (in contrast to the 1st stage), the mother's position does not affect duration or mode of delivery or maternal or neonatal outcome in deliveries without epidural anesthesia (3). Also, the pushing technique (spontaneous versus directed and delayed versus immediate) does not affect the mode of delivery or maternal or neonatal outcome. Use of epidural anesthesia delays pushing and may lengthen the 2nd stage by an hour (4).
The 3rd stage of labor begins after delivery of the infant and ends with delivery of the placenta. This stage usually lasts only a few minutes but may last up to 30 minutes.