NORMAL
BIRTH
 |
| CARDINAL MOVEMENTS |
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.
