Labor generally begins between the 38th and 42nd week of gestation. Prior to the onset of labor, physicochemical changes occur in the cervix and are collectively called “ripening.” When this is combined with the increasingly frequent and strong uterine contractions of late pregnancy, the cervix begins the process of effacement. In a proposed self-perpetuating process, effacement of the cervix results in the production and liberation of more prostaglandins, further stimulating uterine contractions (Ferguson reﬂex). Cervical effacement is common before the onset of true labor.
Labor is deﬁned by rhythmic uterine contractions that result in progressive effacement and dilation of the cervix. Although labor is a continuous process, it is divided into four functional stages: (1) ﬁrst stage between the onset of labor and full cervical dilation (10 cm); (2) second stage from complete cervical dilation through the delivery of the infant; (3) third stage from immediately after delivery to delivery of the placenta; and (4) fourth stage the 2 hours after delivery of the placenta, during which time there is signiﬁcant physiologic alteration. The ﬁrst stage of labor is further subdivided into the latent and active phases, demarcated by cervical dilation of roughly 3 to 5 cm and an accelerated rate of cervical change. The average duration of labor for ﬁrst-time mothers is approximately 9 hours and 6 hours for multiparous women. The upper limit (95th percentile) of labor duration is roughly 18 and 13 hours, respectively.
Once the cervix is completely dilated, the fetus (in the vertex position) must descend though the vagina in a series of six cardinal movements ending in delivery. These are engagement, ﬂexion, descent, internal rotation, extension, and external rotation. Engagement of the fetal head and some descent may occur before complete dilation has been accomplished. Engagement is deﬁned as descent of the fetal biparietal diameter to below the pelvic inlet, identiﬁed clinically by the presence of the presenting part below the level of ischial spines (0 station). Flexion of the fetal head allows for the smaller diameters of the fetal head to present to the maternal pelvis. Descent is a necessity for the successful completion of passage through the vagina. Internal rotation, like ﬂexion, facilitates presentation of the optimal diameters of the fetal head to the bony pelvis, most commonly rotating from transverse to either occiput anterior or posterior. Extension of the fetal head occurs as it reaches the introitus and accommodates the upward curve of the birth canal at its distal end. External rotation occurs after delivery of the head as the head restitutes relative to the shoulders. These cardinal movements do not occur as a distinct series of movements but rather as a group of movements that overlap as the fetus moves progressively toward delivery.
During the course of labor, the well-being of both mother and baby must be evaluated by periodic assessment of the mother’s vital signs and the fetal heart rate. The latter may be accomplished by either intermittent auscultation after contractions or by continuous electronic fetal monitoring devices. Maternal hydration is most often maintained by intravenous ﬂuids because of limited or absent gastric emptying that occurs during labor. Amelioration of pain may be accomplished by systemic analgesics early in labor or by regional anesthetics as labor progresses.
Following the delivery of the placenta, the uterus must contract to prevent maternal hemorrhage. To accomplish this, the use of uterine massage as well as uterotonic agents such as oxytocin, methylergonovine maleate, or prostaglandins may be routinely used. Excessive blood loss at this or any subsequent time should suggest the possibility of uterine atony, uterine inversion, or unrecognized cervical, vaginal, or other laceration.