Objective: To compare the normal labor progress of women whose labor was induced with that of women who labored spontaneously.
Methods: A retrospective cohort study of all consecutive women admitted for labor at 37 weeks or more of gestation from 2004-2008 who reached the second stage of labor. Women whose labor was induced and women whose labor was augmented were compared with women who labored spontaneously without augmentation. Results were stratified by parity. Univariable and multivariable analyses were performed; interval censored regression was used to estimate the median time spent to progress 1 cm in dilation and the total time from 4-10 cm dilation by parity.
Results: Of 5,388 women in the cohort, 2,021 spontaneously labored, 1,720 were augmented, and 1,647 were induced. After adjusting for race, obesity, macrosomia, and Bishop score, women who were induced had a significantly longer total time in labor than women who labored spontaneously (median [95 percentile] in hours for nulliparous women: 5.5 [16.8] induced compared with 3.8 [11.8] spontaneous; for multiparous women 4.4 [16.2] induced compared with 2.4 [8.8] spontaneous). However, median time to progress 1 cm dilation in active labor (6 cm or greater) was similar in induced and spontaneous labor. The time to progress 1 cm dilation in latent labor (less than 6 cm) was significantly longer in women who were induced compared with women who experienced spontaneous labor.
Conclusion: The latent phase of labor is significantly longer in induced labor compared with spontaneous labor, although the active phase of labor (greater than 6 cm) is similar between the two groups. Arrest diagnoses before 6 cm in women undergoing induction should be made cautiously.
Level of evidence: II.