Labor dystocia refers to a slow and abnormal progression of labor. It is the most common problem associated with labor and primarily affects nulliparous women. A Danish study reported a 37% incidence of dystocia among uncomplicated pregnancies in nulliparous women. Labor dystocia is the leading indication for primary cesarean sections.
The definition of dystocia is based on the deviations from the normal labor curve established by Friedman. Because it is difficult in many cases to be certain exactly when labor began, dystocia is rarely diagnosed with absolute certainty. A commonly accepted definition of dystocia is a rate of cervical dilatation less than 0.5 cm/hr over 4 hours in the active phase of the first stage of labor or fetal descent of less than 1 cm/hr in the second stage. These definitions are based on the 95th percentile for duration of labor in low-risk women with spontaneous labor. The term failure to progress has also been commonly used and refers to either a lack of progressive cervical dilatation or a lack of fetal descent or both.
ETIOLOGY AND RISK FACTORS
The principal causes of dystocia are related to the 4 Ps: power, passenger, passage, and psyche.
Uterine contractions may be infrequent, hypotonic, or incoordinate such that they are unable to dilate the cervix. This is commonly seen in primary dysfunctional labor. Normal uterine contractions in the active phase have been defined as uterine contraction pressures greater than 200 Montevideo units. Maternal exhaustion or dense motor blockage from regional anesthesia can result in ineffective maternal expulsive efforts in the second stage.
Fetal malposition and malpresentation (e.g., asynclitism, persistent occiput posterior, brow presentation) are associated with dystocia. If the fetus is disproportionately large relative to the maternal pelvis or if there is a congenital anomaly (hydrocephalus), prolonged labor can also ensue.
Examination of the pelvis may reveal an inadequate pelvis. Any prominent ischial spines, a narrow pubic arch, or other soft tissue mass (e.g., tumors, septums) may impede progressive descent of the fetus. True cephalopelvic disproportion refers to a disparity between the pelvic architecture or size and the fetal head that precludes vaginal delivery.
Inlet contraction is present when the anteroposterior diameter (obstetric conjugate) is less than 10 cm or the transverse diameter is less than 12 cm. Inlet contraction may result from rickets or from generally poor development.
Effects on the fetus are:
Failure of engagement
Increase in malpositions
Formation of a large caput succedaneum
Prolapse of the umbilical cord. This becomes a complication because the presenting part does not fit the inlet well