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… pains become less frequent and less intense, although giving rise to quite as much or even more suffering than previously. At the same time, the cervix, which was becoming obliterated and dilated in a satisfactory manner, ceases to make further progress and labour apparently comes to a standstill.
—J. Whitridge Williams (1903)
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The term dystocia as described by Williams in the first edition of this text still applies today. It literally means difficult labor and is characterized by abnormally slow labor progress. Similar to the factors described by Williams, dystocia arises from three distinct abnormality categories. First, uterine contractions may be insufficiently strong or inappropriately coordinated to efface and dilate the cervix—uterine dysfunction. Also, voluntary maternal muscle effort during second-stage labor may be inadequate. Second, fetal abnormalities of presentation, position, or anatomy may slow progress. Last, structural changes can contract the maternal bony pelvis. Or, soft tissue abnormalities of the reproductive tract may form an obstacle to fetal descent. More simply, these alterations can be mechanistically simplified into three categories that include abnormalities of the powers—uterine contractility and maternal expulsive effort; of the passenger—the fetus; and of the passage—the pelvis and lower reproductive tract.
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Abnormalities shown in Table 23-1 often interact singly or in combination to produce dysfunctional labor. Commonly used expressions today such as cephalopelvic disproportion and failure to progress are used to describe ineffective labors. Of these, cephalopelvic disproportion is a term that came into use before the 20th century to describe obstructed labor resulting from disparity between the fetal head size and maternal pelvis. But, the term originated at a time when the main indication for cesarean delivery was overt pelvic contracture due to rickets (Olah, 1994). Such absolute disproportion is now rare, and most cases result from malposition of the fetal head within the pelvis (asynclitism) or from ineffective uterine contractions. True disproportion is a tenuous diagnosis because many women who undergo cesarean delivery for this reason subsequently deliver even larger newborns vaginally in subsequent pregnancies. 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 lack of fetal descent. Neither of these two terms is specific.
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Mechanisms of Dystocia
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At the end of pregnancy, the fetal head encounters a relatively thick lower uterine segment and undilated ...