FIGURE22-17 Average labor curves for women with singleton term pregnancies presenting in spontaneous labor with vaginal delivery for nulliparas from 1959-1966 compared with those from 2002-2008.
(Redrawn from Laughon SK, Branch W, Beaver J, et al: Changes in labor patterns over 50 years. Am J Obstet Gynecol 206:419.e1.9, 2012.) at any time during pregnancy. False labor often stops spontaneously, or it may proceed rapidly into efective contractions.
A second method deines the onset of labor as beginning at the time of admission to the labor unit. In the United States, admission for labor is frequently based on the extent of cervical dilation accompanied by painful contractions. If a woman has intact membranes, then a cervical dilation of3 to 4 cm or greater is presumed to be a reasonably reliable threshold for the diagnosis of labor. In this case, labor onset commences with the time of admission. his presumptive method obviates many of the uncertainties in diagnosing labor during earlier stages of cervical dilation. Laughon and associates (2012) compared the duration of spontaneous labor at term in nulliparas delivered in the United States between 1959 and 1966 to that of those delivered from 2002 to 2008. As shown in Fiure 22-17, during those 50 years, the length oflabor increased by approximately 2 hours.
• First Stage of Labor
For labor, Friedman (1954) described a characteristic sigmoid pattern by graphing cervical dilationagainst time. his graphical approach, which was based on statistical observations, changed labor management. Friedman developed the concept of three functional labor divisions to describe the physiological objectives ofeach division (Fig. 22-18). First, during the preparatoy division, although the cervix dilates little, its connective tissue components change considerably (Chap. 21, p. 413). Sedation and conduction analgesia are capable of arresting this labor division. he dilationaldivision, duringwhich dilation proceeds at its most rapid rate, is unafected by sedation. Last, the pelvic division commences with the deceleration phase of cervical dilation. The classic labor mechanisms that involve the cardinal fetal movements of the cephalic presentation take place principally during this pelvic division. In actual practice, however, the onset of the pelvic division is seldom clearly identiiable.
As shown in Figure 22-18, the pattern of cervical dilation during the preparatory and dilational divisions of normal labor is a sigmoid curve. Two phases of cervical dilation are deined. he latent phase corresponds to the preparatory division, and 0i::'a� ofdilatation and descent curves into: (1) a preparatory division, including latent and acceleration phases; (2) a dilatational division, occupying the phase of maximum slope; and (3) a pelvic division, encompassing both deceleration phase and second stage concurrent with the phase of maximum slope of descent. (Redrawn from Friedman EA: Labor: Clinical Evaluation and Management, 2nd ed. New York, Appleton-Century-Crofts, 1978.) the activephase to the dilational division. Friedman further subdivided the active phase into the acceleration phase, thephase of mximum slope, and the deceleration phase (Fig. 22-19).
he onset of latent labor, as deined by Friedman (1972), is the pointatwhich the motherperceives regularcontractions. helatent phase for most women ends once dilation of3 to 5 cm is achieved. his threshold may be clinically useul, for it deines dilation limits beyond which active labor can be expected. More recently, a Consensus Committee of the American College of Obstetricians and ::0'a� 0 1)':D