Abnormal birth mechanism

Failure of adequate progression of labor due to cephalopelvic disproportion with imminent fetal asphyxia is the most common reason to perform secondary cesarean section. Arrest is typically caused by a combination of a large infant, an abnormal birth mechanism, and a narrow maternal pelvis. Detectable abnormal narrowing with an absolute disproportion occurs in 0.5%-l% of all deliveries today (Fig. 14.1). The incidence of borderline pelvic findings in which the size of the child and the birth mechanism together decide whether spontaneous delivery will be possible is much higher (Table 14.1). 

An abnormal birth mechanism (abnormal fetal presentation and position) prevents adequate progression of labor just as often as maternal factors. Other fetal causes are macrosomia or fetal anomalies asso-... 

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