Hendricks and coworkers (1970) challenged Friedman's conclusions about the course of normal human labor. Their principal diferences included: (1) absence of a latent phase, (2) no deceleration phase, (3) brevity ofilabor, and (4) dilation at similar rates for nulliparas and multiparas after 4 cm. They disputed the concept of a latent phase because they observed that the cervix dilated and efaced slowly during the 4 weeks preceding labor. They contended that the latent phase actually progressed over several weeks. hey also reported that labor was relatively rapid. Specifically, the average time from admission to complete dilation was 4.8 hours for nulliparas and 3.2 hours for multiparas.
Others have reassessed the Friedman labor curves. Zhang and associates (2010) studied electronic labor records from 62,415 parturients with spontaneous labor at term and vaginal birth. For nulliparas, the median time to progress from 4 to 5 cm was 1.3 hours, from 5 to 6 cm 0.8 hours, and thereafter, additional centimeters were gained approximately each 0.5 hours. They found that normal labor may take more than 6 hours to progress from 4 to 5 cm and more than 3 hours to progress from 5 to 6 cm dilation. Rates for multiparas were similar from 4 to 6 cm. Then, labor accelerated much faster in multiparas. Data from this study form the foundation for new guidelines regarding cesarean delivery indications for labor arrest put forth in the Obstetric Care Consensus document by the American College of Obstetrics and Gynecology and Society for Maternal-Fetal Medicine (2016c) and described in Chapter 23 (p. 444).
In a study performed at Parkland Hospital, epidural analgesia was found to lengthen the active phase of the Friedman labor curve by 1 hour (lexander, 2002). his increase was the result of a slight but significant decline in the rate of cervical dilation-1.4 cm/hr in women given epidural analgesia compared with 1.6 cm/hr in those without such analgesia. Several other reports also note that maternal obesity lengthens the irst stages oflabor by 30 to 60 minutes (Chin, 2012; Kominiarek, 2011). Finally, Adams and coworkers (2012) found that maternal fear prolonged labor by approximately 45 minutes.
Active-phase abnormalities have been reported to occur in 25 percent of nulliparous and 15 percent of multiparous labors (Sokol, 1977). Friedman (1972) subdivided active-phase problems into protraction and arrest disorders. Abnormal labor patterns, diagnostic criteria, and treatment methods are summarized in Chapter 23 (p. 442).
• Second Stage of Labor his stage begins with complete cervical dilation and ends with fetal delivery. The median duration is approximately 50 minutes for nulliparas and about 20 minutes for multiparas, but it is highly variable (Kilpatrick, 1989). In a woman of higher parity with a previously dilated vagina and perineum, two or three expulsive eforts after full cervical dilation may suice to complete delivery. Conversely, in a woman with a contracted pelvis, with a large fetus, or with impaired expulsive eforts from conduction analgesia or sedation, the second stage may be longer. Higher maternal body mass index does not interfere with second-stage labor length (Carlhall, 2013; Robinson, 2011). Abnormalities of this labor stage are described in Chapter 23 (p. 446).