Definition
Labor is the physiologic process by which a fetus is expelled from the uterus to the outside world. It is a clinical diagnosis requiring two elements: (1) regular phasic uterine contractions increasing in frequency and intensity, and (2) progressive effacement and dilation of the cervix. Normal labor occurs at term (defined as 37–0/7 to 42–0/7 weeks’ gestation).
The endocrine control of labor
Labor may be regarded physiologically as a release from the inhibitory effects of pregnancy on the myometrium rather than as an active process mediated by uterine stimulants.
In vivo, however, both inhibitory and stimulatory factors appear to be important. There is likely a “parturition cascade” at term that removes the mechanisms maintaining uterine quiescence and recruits factors promoting uterine activity.
Regardless of whether the trigger for labor begins within the fetus or outside the fetus, the final common pathway ends in the maternal tissues of the uterus and is characterized by the development of regular phasic uterine contractions.
Myometrial Contractility
As in other smooth muscles, myometrial contractions are mediated through the ATP-dependent binding of thick filaments (myosin) to thin filaments (actin) (Figure 60.1). Electrical stimuli (action potentials) must be generated and propagated in the myometrium to cause contractions, which are achieved through the rapid shift of ions (especially calcium) through membrane ion channels. The frequency of contractions correlates with the frequency of action potentials, the force of the contractions correlates with the number of spikes in the action potential and the number of cells activated together, and the duration of contractions correlates with the duration of the action potentials. The transition of the uterus from a quiescent entity to a contractile one comes in part through an increase in gap junctions leading to recruitment and improved communication between adjacent myometrial cells.
In contrast to vascular smooth muscle, myometrial cells have a sparse innervation which is further reduced during pregnancy. The regulation of uterine contractility is therefore largely humoral and/or dependent on intrinsic factors within myometrial cells.
Mechanics of normal labor
The ability of the fetus to successfully negotiate the pelvis is dependent on the interaction of three variables (known as “the 3 Ps”): powers, passenger, and passage.
The “powers” refers to the forces generated by the uterine musculature, the “passenger” is the fetus,
and the “passage” consists of the bony pelvis and resistance provided by soft tissues, specifically the cervix and pelvic floor musculature.
Powers
• Several techniques are available to assess uterine activity. Uterine activity is characterized by frequency, intensity (amplitude), and duration of contractions.
• Despite technological advances, the definition of “adequate” uterine activity remains unclear. Classically, three to five strong contractions in 10 minutes has been used to define adequate labor. This contraction pattern is seen in 95% of women in normal labor at term. Remember that the external uterine monitor is a tonometer (it measures muscle tone). It provides an accurate measure of the timing of contractions, but not the intensity. If an intrauterine pressure catheter (IUPC) is used, 150–200 Montevideo units (strength of contractions in millimeters of mercury multiplied by the frequency per 10 minutes) are deemed adequate. The ultimate barometer of uterine activity is the rate of cervical dilation and descent of the presenting part.
Passenger
• Two main fetal variables influence the course of labor: fetal size and attitude (degree of flexion or extension of the head). When the fetal head is optimally flexed, the smallest possible diameter (suboccipitobregmatic diameter 9.5 cm) presents at the pelvic inlet.
• The lie, presentation, position, and station of the fetus can be assessed on clinical examination. Lie refers to the long axis of the fetus relative to the long axis of the uterus and can be longitudinal, transverse, or oblique. Presentation can be either cephalic or breech, referring to the pole of the fetus that overlies the pelvic inlet. Position refers to the relationship of a nominated site on the presenting part of the fetus to a nominated location on the maternal pelvis and can be assessed most accurately on bimanual examination. In a cephalic presentation, the nominated site is usually the occiput. In the breech, the nominated site is the sacrum. Station refers to the leading bony edge of the presenting part relative to the maternal pelvis (specifically the ischial spines) as assessed on bimanual examination. The vertex is said to be engaged when the widest diameter has entered the pelvic inlet, which is best assessed on the abdominal examination.
• Fetal weight can be estimated clinically or by ultrasound. Using birth weight as the gold standard, both techniques are equally accurate with an error of 15–20%.
Passage
• The bony pelvis is composed of the sacrum, ilium, ischium, and pubis. The shape of the pelvis can be classified into one or more four broad categories: gynaecoid, android, anthropoid, and platypelloid The gynaecoid pelvis is the classic female shape.
• Clinical pelvimetry can be used to estimate the shape and adequacy of the bony pelvis but has not been shown to accurately predict the course of labor or to change clinical management.
• Pelvic soft tissues (cervix and pelvic floor musculature) can provide resistance in labor. In the second stage, the pelvic musculature may play an important role in facilitating rotation and descent of the head. Excessive resistance may, however, contribute to failure to progress in labor.
Stages of labor
Labor is a continuous process. For clinical purposes, however, it is divided into three stages:
1. The first stage refers to the interval between the onset of labor and full cervical dilation. It is further divided into the latent phase (the period between the onset of labor and a point at which a change in the slope of the rate of cervical dilation is noted) and the active phase (which is associated with a greater rate of cervical dilation and usually beings at around 3–4 cm dilation).
The partogram (Friedman curve) is a graphic representation of the normal labor curve against which a patient’s progress is plotted. The normal latent phase is <20 hours in nullipara and <14 hours in multipara. Inactive phase, the cervix should dilate a minimum of >1.2 cm/h in nullipara (>1.5 cm/h in multipara). A delay in cervical dilation in the active phase of ≥2 hours over that expected suggests labor dystocia and requires further evaluation.
2. The second stage commences when the cervix achieves full dilation (10 cm) – not when the mother starts to push – and ends with the delivery of the fetus. Prolonged second stage refers to >3 hours with or >2 hours without regional analgesia in a nullipara and >2 hours with or >1 hour without regional analgesia in a multipara.
3. The third stage refers to the delivery of the placenta and fetal membranes and usually lasts <10 min. In the absence of excessive bleeding, up to 30 min may be allowed before intervention.
Cardinal movements in normal labor
The cardinal movements refer to the changes in position of fetal head required for the fetus to successfully negotiate the birth canal and include the following:
1 Engagement refers to the passage of the widest diameter of the presenting part to a level below the plane of the pelvic inlet. In a cephalic fetus with a well-flexed head, the largest transverse diameter is the biparietal diameter (9.5 cm). In nulliparas, engagement of the fetal head usually occurs by 36 weeks. Failure of the head to engage by this time may be a sign of cephalopelvic disproportion (CPD). In multipara, engagement can occur later or even during labor.
2 Descent refers to the downward passage of the presenting part through the pelvis.
3 Flexion of the fetal head on to the chest occurs passively as it descends due to the shape of the bony pelvis and the resistance of the pelvic floor. Although flexion of the head is present to some degree in most fetuses before labor, complete flexion occurs only during labor. The result of complete flexion is to present the smallest diameter of the fetal head (suboccipitobregmatic diameter) for optimal passage through the pelvis.
4 Internal rotation refers to rotation of the presenting part from its original position (transverse with regard to the birth canal) to the anteroposterior position as it passes through the pelvis. As with flexion, internal rotation is a passive movement resulting from the shape of the pelvis and the pelvic floor musculature. As the head descends, the occiput of the fetus rotates towards the symphysis pubis (or, less commonly, towards the hollow of the sacrum), thereby allowing the widest portion of the fetus to negotiate the pelvis at its widest dimension. Due to the angle of inclination between the maternal lumbar spine and pelvic inlet, the fetal head engages in an asynclitic fashion (ie, with one parietal eminence lower than the other). With uterine contractions,
the leading parietal eminence descends and is first to engage the pelvic floor. As the uterus relaxes, the pelvic floor musculature causes the fetal head to rotate until it is no longer asynclitic.
5 Extension occurs once the fetus has descended to the level of the introitus. This descent brings the base of the occiput into contact with the inferior margin at the symphysis pubis. At this point, the birth canal curves upwards. The fetal head is delivered by extension and rotates around the symphysis pubis. The forces responsible for this motion are the downward force exerted on the fetus by the uterine contractions along with the upward forces exerted by the muscles of the pelvic floor.
6 External rotation, also known as restitution, refers to the return of the fetal head to the correct anatomic position in relation to the fetal torso. This can occur to either side depending on the orientation of the fetus. This is again a passive movement resulting from a release of the forces exerted on the fetal head by the maternal bony pelvis and its musculature and mediated by the basal tone of the fetal musculature.
7 Expulsion refers to the delivery of the rest of the fetus. After delivery of the head and external rotation, further descent brings the anterior shoulder to the level of the symphysis pubis. The anterior shoulder is delivered in much the same manner as the head, with rotation of the shoulder under the symphysis pubis. After the shoulder, the rest of the body is usually delivered without difficulty.
Clinical assistance at delivery
The goals of clinical assistance at delivery are to support the mother psychologically, reduce maternal trauma, prevent fetal injury, and resuscitate the newborn if required.
• As the fetal head crowns, the clinician’s hand is used to control delivery and prevent precipitous expulsion (which has been associated with perineal injury in the mother and intracranial hemorrhage in the neonate).
• Mouth and pharynx can be gently suctioned, although this maneuver has largely fallen out of favor as it has not been shown to change perinatal outcome. Vigorous suctioning can cause a vagal response and fetal bradycardia and should be avoided.
• If a nuchal cord is present, it should be reduced at this time.
• Following restitution of the fetal head, a hand is placed on each parietal eminence and the anterior shoulder delivered by gentle downward traction.
• The posterior shoulder and torso are then delivered by upward traction.
• The umbilical cord should be double clamped and cut. Delayed cord clamping has been shown to increase blood flow to the infant and thus increase its hematocrit. It may have clinical benefit in preterm infants, but has not been shown to significantly improve perinatal outcome at term.
• The infant should be supported at all times.
• The third stage of labor can be managed either passively or actively.
• The placenta and fetal membranes should be examined, and the number of blood vessels in the umbilical cord recorded. If indicated, the placenta should be sent for pathologic examination.