FIGURE 22-19 Composite ofthe average dilatation curve for nulliparous labor. The first stage is divided into a relatively flat latent phase and a rapidly progressive active phase. In the active phase, there are three identifiable component parts that include an acceleration phase, a phase ofmaximum slope, and a deceleration phase. (Redrawn from Friedman EA: Labor: Clinical Evaluation and Management, 2nd ed. New York, Appleton-Century-Crofts, 1978.) -Hendricks, Brenner & Kraus Ledger
Univ. of Michigan -Rodesch et al. Temple Univ.
FIGURE 22-20 Progress of labor in primigravid women from the time of admission. When the starting point on the abscissa begins with admission to the hospital, a latent phase is not observed.
Gynecologists and Society for Maternal-Fetal Medicine (2016c) has redefined active labor to begin at 6 cm. A uller discussion of these labor changes is found in Chapter 23 (p. 445).
his concept of a latent phase has great signiicance in understanding normal human labor, because labor is considerably longer when a latent phase is included. To better illustrate this, labor was diagnosed beginning with their admission, rather than with the onset of regular contractions. When labor is deined similarly, individual labor curves are remarkably comparable.
Aprolonged latent phase was defined by Friedman and Sachtleben (1963) as one exceeding 20 hours in the nullipara and 14 hours in the multipara. hese times corresponded to the 95th percentiles. Factors that afected latent phase duration include excessive sedation or epidural analgesia; unfavorable cervical condition, that is, thick, unefaced, or undilated; and false labor. Of women who had been administered heavy sedation, 85 percent eventually entered active labor. In another 10 percent, uterine contractions ceased, suggesting that they had false labor. The remaining 5 percent experienced persistence of an abnormal latent phase and required oxytocin stimulation. Amniotomy was discouraged because of the 10-percent incidence of false labor. Sokol and associates (1977) reported a 3-to 4-percent incidence of prolonged latent phase, regardless of parity. Friedman (1972) reported that latent phase prolongation did not adversely inluence fetal or maternal morbidity or mortality rates. However, Chelmow and coworkers (1993) disputed the long-held belief that prolongation of the latent phase is benign.
The progress of labor in nulliparas has particular significance because these curves all reveal a rapid change in the slope of cervical dilation rates between 3 and 5 cm (see Fig. 22-20).
Thus, cervical dilation of3 to 6 cm or more, in the presence of uterine contractions, can be taken to reliaby represent the threshold or active labor. Similarly, these curves provide useful guideposts for labor management.
Turning again to Friedman (1955), the mean duration of active-phase labor in nulliparas was 4.9 hours. But, the standard deviation of 3.4 hours is large, hence, the active phase was reported to have a statistical maximum of 11.7 hours. Indeed, rates of cervical dilation ranged from a minimum of 1.2 up to 6.8 cm/hr. Friedman (1972) also found that multiparas progress somewhat faster in active-phase labor, with a minimum normal rate of 1.5 cm/hr. His analysis of active-phase labor concomitantly describes rates of fetal descent and cervical dilation (see Fig. 22-18). Descent begins in the later stage of active dilation, commencing at 7 to 8 cm in nulliparas and becoming most rapid after 8 cm.