Labor arrest, abnormal fetal presentation, or fetal jeopardy are indications for a large percentage of primary cesarean deliveries in the United States (Boyle, 2013). Lowering dystocia rates offers the potential to decrease rates of this surgery and associated maternal morbidity.
Dystocia literally means difficult labor and is characterized by abnormally slow labor progress. Causes are grouped into three distinct categories. Mechanistically, these simplify into abnormalities of the powers—poor uterine contractility and maternal expulsive effort; of the passenger—the fetus; and of the passage—the pelvis and lower reproductive tract.
These three groups act singly or in combination to produce dysfunctional labor (Table 23-1). For the powers, uterine contractions may be insufficiently strong or inappropriately coordinated to efface and dilate the cervix. This is termed uterine dysfunction. Moreover, during second-stage labor, voluntary maternal pushing may be inadequate. For the passenger, fetal abnormalities of presentation, position, or anatomy may slow progress. Last, for the passage, structural changes can contract the maternal bony pelvis. Or, soft tissue abnormalities of the reproductive tract may block fetal descent.
TABLE 23-1Some Causes of Dystocia in Term Vertex Singletons ||Download (.pdf) TABLE 23-1Some Causes of Dystocia in Term Vertex Singletons
|Fetal characteristics |
|Presentation: face, brow, sinciput |
|Position: OT, OP, asynclitism |
|Anomaly: sacrococcygeal teratoma, hydrocephalus, craniofacial tumor, anencephaly |
|Intrapartum findings |
|Neuraxial analgesia |
|Higher station at labor onset |
|Poor maternal pushing: sedation, severe pain, dense regional block, neurologic disease |
|Maternal characteristics |
|Increasing age |
|Large leiomyoma |
|Uterine müllerian anomaly |
|Anthropoid, android, or platypelloid pelvis types |
|Narrow pelvic diameters |
|Short stature |
|Pelvic tumor |
|Prior pelvic fracture |
To describe ineffective labors, two commonly used terms are cephalopelvic disproportion (CPD) and failure to progress. CPD describes obstructed labor resulting from disparity between the fetal head size and maternal pelvis. The term CPD originated at a time when the main indication for cesarean delivery was overt pelvic contracture from rickets (Olah, 1994). Such absolute disproportion is now rare, and most cases result from malposition of the fetal head within the pelvis (asynclitism). True disproportion is a tenuous diagnosis because 50 to 75 percent of women undergoing cesarean delivery for this reason subsequently deliver even larger newborns vaginally (Lewkowitz, 2015; Place, 2019).
A second phrase, failure to progress in either spontaneous or stimulated labor, has become an increasingly popular description of ineffectual labor. This term reflects lack of progressive cervical dilation or halted fetal descent.
ABNORMALITIES OF THE EXPULSIVE FORCES
Types of Uterine Dysfunction
Uterine contractions are needed to dilate the cervix and to expel the fetus. A contraction is initiated by spontaneous action potentials in the membrane of smooth muscle cells. Unlike the ...