he mechanism by which the biparietal diameter-the greatest transverse diameter in an occiput presentation-passes through the pelvic inlet is designated engagement. he fetal head may engage during the last few weeks of pregnancy or not until after labor commencement. In many multiparas and some nulliparas, the fetal head is freely movable above the pelvic inlet at labor onset. In this circumstance, the head is sometimes referred to as "floating." A normal-sized head usually does not engage with its sagittal suture directed anteroposteriorly. Instead, as discussed, the fetal head usually enters the pelvic inlet either transversely or obliquely. Segel and coworkers (2012) analyzed labor in 5341 nulliparous women and found that fetal head engagement before labor onset did not afect vaginal delivery rates in either spontaneous or induced labor.
The fetal head tends to accommodate to the transverse axis of the pelvic inlet, whereas the sagittal suture, while remaining parallel to that axis, may not lie exactly midway between the symphysis and the sacral promontory. The sagittal suture frequently is deflected either posteriorly toward the promontory or anteriorly toward the symphysis (Fig. 22-12). Such lateral delection to a more anterior or posterior position in the pelvis is called asynclitism. If the sagittal suture approaches the sacral promontory, more of the anterior parietal bone presents itself to the examining fingers, and the condition is called anterior asynclitism. If, however, the sagittal suture lies close to the symphysis, more of the posterior parietal bone will present, and the condition is called posterior asynclitism. With extreme posterior asynclitism, the posterior ear may be easily palpated.
Moderate degrees of asynclitism are the rule in normal labor.
However, if severe, the condition is a common reason for cepha lopelvic disproportion even with an otherwise normal-sized pelvis. Successive fetal head shiting from posterior to anterior asynclitism aids descent.
This movement is the first reqUISIte for birth of the newborn. In nulliparas, engagement may take place before the onset oflabor, and further descent may not follow until the onset of the second stage. In multiparas, descent usually begins with engagement. Descent is brought about by one or more of four forces: (1) pressure of the amnionic fluid, (2) direct pressure of the fundus upon the breech with contractions, (3) bearing-down eforts of maternal abdominal muscles, and (4) extension and straightening of the fetal body.
1.Head floating, before engagement 5.Complete extension 2.Engagement, descent, flexion 6.Restitution (external rotation) 3.Further descent, internal rotation 7.Delivery of anterior shoulder
FIGURE 22-11 Cardinal movements of labor and delivery from a left occiput anterior position.
4.Complete rotation, beginning extension 8.Delivery of posterior shoulder
Anterior asynclitism Normal synclitism Posterior asynclitism '\ '\, ,/�>,I '.-Occipito-Sagittal � frontal plane -_.. Anterior parietal suture Pelvic inlet plane Posterior parietal
FIGURE 22-12 Synclitism and asynclitism.
As soon as the descending head meets resistance, whether from the cervix, pelvic walls, or pelvic loor, it normally lexes. With this movement, the chin is brought into more intimate contact with the fetal thorax, and the appreciably shorter suboccipitobregmatic diameter is substituted for the longer occipitofrontal diameter (Fig. 22-13).
his movement turns the occiput gradually away from the transverse axis. Usually the occiput rotates anteriorly toward the symphysis pubis, but less commonly, it may rotate posteriorly