Labor is a physiologic process during which the products of conception (ie, the fetus, membranes, umbilical cord, and placenta) are expelled outside of the uterus. Labor is achieved with changes in the biochemical connective tissue and with gradual effacement and dilatation of the uterine cervix as a result of rhythmic uterine contractions of sufficient frequency, intensity, and duration. 1,2 Labor is a clinical diagnosis. The onset of labor is defined as regular, painful uterine contractions resulting in progressive cervical effacement and dilatation.
Cervical dilatation in the absence of uterine contraction suggests cervical insufficiency, whereas uterine contraction without cervical change does not meet the definition of labor. Stages of Labor and Epidemiology Stages of Labor Obstetricians have divided labor into 3 stages that delineate milestones in a continuous process. First stage of labor The first stage begins with regular uterine contractions and ends with complete cervical dilatation at 10 cm.
In Friedman’s landmark studies of 500 nulliparas3 , he subdivided the first stage into an early latent phase and an ensuing active phase. The latent phase begins with mild, irregular uterine contractions that soften and shorten the cervix. The contractions become progressively more rhythmic and stronger. This is followed by the active phase of labor, which usually begins at about 3-4 cm of cervical dilation and is characterized by rapid cervical dilation and descent of the presenting fetal part. The first stage of labor ends with complete cervical dilation at 10 cm.
According to Friedman, the active phase is further divided into an acceleration phase, a phase of maximum slope, and a deceleration phase. Characteristics of the average cervical dilatation curve is known as the Friedman labor curve, and a series of definitions of labor protraction and arrest were subsequently established. 4,5 However, subsequent data of modern obstetric population suggest that the rate of cervical dilatation is slower and the progression of labor may be significantly different from that suggested by the Friedman labor curve. ,7,8 Second stage of labor The second stage begins with complete cervical dilatation and ends with the delivery of the fetus.
The AmericanCollege of Obstetricians and Gynecologists (ACOG) has suggested that a prolonged second stage of labor should be considered when the second stage of labor exceeds 3 hours if regional anesthesia is administered or 2 hours in the absence of regional anesthesia for nulliparas. In multiparous women, such a diagnosis can be made if the second stage of labor exceeds 2 hours with regional anesthesia or 1 hour without it. Studies performed to examine perinatal outcomes associated with a prolonged second stage of labor revealed increased risks of operative deliveries and maternal morbidities but no differences in neonatal outcomes. 9,10,11,12 Maternal risk factors associated with a prolonged second stage include nulliparity, increasing maternal weight and/or weight gain, use of regional anesthesia, induction of labor, fetal occiput in a posterior or transverse position, and increased birthweight. 11,12,13,14 Third stage of labor.
The third stage of labor is defined by the time period between the delivery of the fetus and the delivery of the placenta and fetal membranes. During this period, uterine contraction decreases basal blood flow, which results in thickening and reduction in the surface area of the myometrium underlying the placenta with subsequent detachment of the placenta. 15 Although delivery of the placenta often requires less than 10 minutes, the duration of the third stage of labor may last as long as 30 minutes. Expectant management of the third stage of labor involves spontaneous delivery of the placenta.
Active management often involves prophylactic administration of oxytocin or other uterotonics (prostaglandins or ergot alkaloids), early cord clamping/cutting, and controlled cord traction of the umbilical cord. A systematic review of the literature that included 5 randomized controlled trials comparing active and expectant management of the third stage reports that active management shortens the duration of the third stage and is superior to expectant management with respect to blood loss/risk of postpartum hemorrhage; however, active management is associated with an increased risk of unpleasant side effects. 6 The third stage of labor is considered prolonged after 30 minutes, and active intervention, such as manual extraction of the placenta, is commonly considered. 2 Mechanism of Labor The ability of the fetus to successfully negotiate the pelvis during labor involves changes in position of its head during its passage in labor. The mechanisms of labor, also known as the cardinal movements, are described in relation to a vertex presentation, as is the case in 95% of all pregnancies.
Although labor and delivery occurs in a continuous fashion, the cardinal movements are described as 7 discrete sequences, as discussed below. 2 Engagement The widest diameter of the presenting part (with a well-flexed head, where the largest transverse diameter of the fetal occiput is the biparietal diameter) enters the maternal pelvis to a level below the plane of the pelvic inlet. On the pelvic examination, the presenting part is at 0 station, or at the level of the maternal ischial spines.
Descent. The downward passage of the presenting part through the pelvis. This occurs intermittently with contractions. The rate is greatest during the second stage of labor. Flexion As the fetal vertex descents, it encounters resistance from the bony pelvis or the soft tissues of the pelvic floor, resulting in passive flexion of the fetal occiput. The chin is brought into contact with the fetal thorax, and the presenting diameter changes from occipitofrontal (11. 0 cm) to suboccipitobregmatic (9. 5 cm) for optimal passage through the pelvis. Internal rotation
As the head descends, the presenting part, usually in the transverse position, is rotated about 45° to anteroposterior (AP) position under the symphysis. Internal rotation brings the AP diameter of the head in line with the AP diameter of the pelvic outlet. Extension With further descent and full flexion of the head, the base of the occiput comes in contact with the inferior margin of the pubic symphysis. Upward resistance from the pelvic floor and the downward forces from the uterine contractions cause the occiput to extend and rotate around the symphysis.
This is followed by the delivery of the fetus’ head. Restitution and external rotation When the fetus’ head is free of resistance, it untwists about 45° left or right, returning to its original anatomic position in relation to the body. Expulsion After the fetus’ head is delivered, further descent brings the anterior shoulder to the level of the pubic symphysis. The anterior shoulder is then rotated under the symphysis, followed by the posterior shoulder and the rest of the fetus.
Clinical History and Physical Examination. History The initial assessment of labor should include a review of the patient’s prenatal care, including confirmation of the estimated date of delivery. Focused history taking should be conducted to include information, such as the frequency and time of onset of contractions, the status of the amniotic membranes (whether spontaneous rupture of the membranes has occurred, and if so, whether the amniotic fluid is clear or meconium stained), the fetus’ movements, and the presence or absence of vaginal bleeding.
Braxton-Hicks contractions, which are often irregular and do not increase in frequency with increasing intensity, must be differentiated from true contractions. Braxton-Hicks contractions often resolve with ambulation or a change in activity. However, contractions that lead to labor tend to last longer and are more intense, leading to cervical change. True labor is defined as uterine contractions leading to cervical changes. If contractions occur without cervical changes, it is not labor. Other causes for the cramping should be diagnosed.
Gestational age is not a part of the definition of labor. In addition, Braxton-Hicks contractions occur occasionally, usually no more than 1-2 per hour, and they often occur just a few times per day. Labor contractions are persistent, they may start as infrequently as every 10-15 minutes, but they usually accelerate over time, increasing to contractions that occur every 2-3 minutes. Patients may also describe what has been called lightening, ie, physical changes felt because the fetus’ head is advancing into the pelvis. The mother may feel that her baby has become light.
As the presenting fetal part starts to drop, the shape of the mother’s abdomen may change to reflect descent of the fetus. Her breathing may be relieved because tension on the diaphragm is reduced, whereas urination may become more frequent due to the added pressure on the urinary bladder. Physical examination Physical examination should include documentation of the patient’s vital signs, the fetus’ presentation, and assessment of the fetal well-being. The frequency, duration, and intensity of uterine contractions should be assessed, particularly the abdominal and pelvic examinations in patients who present in possible labor.
Abdominal examination begins with the Leopold maneuvers described below2 : • The initial maneuver involves the examiner placing both of his or her hands on each upper quadrant of the patient’s abdomen and gently palpating the fundus with the tips of the fingers to define which fetal pole is present in the fundus. If it is the fetus’ head, it should feel hard and round. In a breech presentation, a large, nodular body is felt. • The second maneuver involves palpation in the paraumbilical regions with both hands by applying gentle but deep pressure.
The purpose is to differentiate the fetal spine (a hard, resistant structure) from its limbs (irregular, mobile small parts) to determinate the fetus’ position. • The third maneuver is suprapubic palpation by using the thumb and fingers of the dominant hand. As with the first maneuver, the examiner ascertains the fetus’ presentation and estimates its station. If the presenting part is not engaged, a movable body (usually the fetal occiput) can be felt. This maneuver also allows for an assessment of the fetal weight and of the volume of amniotic fluid. The fourth maneuver involves palpation of bilateral lower quadrants with the aim of determining if the presenting part of the fetus is engaged in the mother’s pelvis. The examiner stands facing the mother’s feet. With the tips of the first 3 fingers of both hands, the examiner exerts deep pressure in the direction of the axis of the pelvic inlet. In a cephalic presentation, the fetus’ head is considered engaged if the examiner’s hands diverge as they trace the fetus’ head into the pelvis. Pelvic examination is often performed using sterile gloves to decrease the risk of infection.
If membrane rupture is suspected, examination with a sterile speculum is performed to visually confirm pooling of amniotic fluid in the posterior fornix. The examiner also looks for fern on a dried sample of the vaginal fluid under a microscope and checks the pH of the fluid by using a nitrazine stick or litmus paper, which turns blue if the amniotic fluid is alkalotic. If frank bleeding is present, pelvic examination should be deferred until placenta previa is excluded with ultrasonography. Furthermore, the pattern of contraction and the patient’s presenting history may provide clues about placental abruption.
Digital examination of the vagina allows the clinician to determine the following: (1) the degree of cervical dilatation, which ranges from 0 cm (closed or fingertip) to 10 cm (complete or fully dilated), (2) the effacement (assessment of the cervical length, which is can be reported as a percentage of the normal 3- to 4-cm-long cervix or described as the actual cervical length); actual reporting of cervical length may decrease potential ambiguity in percent-effacement reporting, (3) the position, ie, anterior or posterior, and (4) the consistency, ie, soft or firm.
Palpation of the presenting part of the fetus allows the examiner to establish its station, by quantifying the distance of the body (-5 to +5 cm) that is presenting relative to the maternal ischial spines, where 0 station is in line with the plane of the maternal ischial spines). 2 The pelvis can also be assessed either by clinical examination (clinical pelvimetry) or radiographically (CT or MRI).
The pelvic planes include the following: • Pelvic inlet: The obstetrical conjugate is the distance between the sacral promontory and the inner pubic arch; it should measure 11. 5 cm or more. The diagonal conjugate is the distance from the undersurface of the pubic arch to sacral promontory; it is 2 cm longer than the obstetrical conjugate. The transverse diameter of the pelvic inlet measures 13. 5 cm. • Midpelvis: The midpelvis is the distance between the bony points of ischial spines, and it typically exceeds 12 cm. Pelvic outlet: The pelvic outlet is the distance between the ischial tuberosities and the pubic arch. It usually exceeds 10 cm. The shape of the mother’s pelvis can also be assessed and classified into 4 broad categories based on the descriptions of Caldwell and Moloy: gynecoid, anthropoid, android, and platypelloid. 24 Although the gynecoid and anthropoid pelvic shapes are thought to be most favorable for vaginal delivery, many women can be classified into 1 or more pelvic types, and such distinctions can be arbitrary
