The relationship between cervical d

The relationship between cervical dilation and the time elapsed in labor is traditionally described by a sigmoid-shaped curve, the “Friedman curve”.1-3 Dilation is divided into a latent phase and an active phase. The latent phase extends from the onset of labor until the upward inflection in the curve and is associated with little incremental change in dilation. According to the Friedman curve, enhancement in the rate of dilation begins at the onset of the active phase, during which most cervical dilation occurs. A gradual increase in dilation (the acceleration phase) initiates the active phase and leads, usually in about an hour, to a period of more rapid and linear dilation (the phase of maximum slope). During the terminal part of the active phase, the deceleration phase, dilation appears to slow. Contemporary labor curves show no deceleration phase, and allow a slower cervical dilation throughout the active phase (Figure 44.1).8 In fact, the cervix continues to open at a constant rate, but as it retracts around the head (which has begun to descend) to achieve complete dilation, its movement is directed cephalolaterally. This change cannot be appreciated readily by examination with the fingers because only radial changes in distance can be determined.