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Friedman’s labor curve defines active phase of labor starting at 4cm cervical dilation. Recent work by Zhang and others suggests that active phase may begin as late as 6cm cervical dilation. Thus, we sought to evaluate the effect on cesarean rates when 6cm as oppose to 4cm cervical dilation is used to define the active phase of labor, given that labor phase sets the standard for arrest diagnoses.
A decision-analytic model was built using TreeAge software and probabilities were derived from the literature. The model compared maternal outcomes when active phase of labor was defined at 4cm as opposed to 6cm cervical dilation. Active phase arrest was strictly defined as no cervical change over two hours of active labor. It was assumed that women with active phase arrest were delivered via cesarean. Primary outcomes investigated were cesarean rates and maternal mortality. Because of heterogeneous labor experiences, the model was stratified by parity and obesity.
Cesarean rates among nulliparous, multiparous, and obese women were higher when active phase was defined at 4cm as compared to 6cm (Table 1). Similar outcomes were seen in maternal mortality rates amongst the subgroups of women investigated.
Redefining the initiation of active phase of labor from 4cm to 6cm substantially decreases rates of cesarean for arrest and subsequent maternal mortality.