The labor curve of the grand multipara: does progress of labor continue to improve with additional childbearing?

OBJECTIVE Our purpose was to test the hypothesis that progress of labor slows as parity exceeds 4 by comparing labor curves of grand multiparous women (para 5 and over) (GMs) with those of nulliparous and lower-parity multiparous women. STUDY DESIGN Retrospective cohorts of spontaneously laboring, vertex-presenting, term GMs who were admitted to two medical centers during the period from January 1990 through June 1995 were randomly computer-matched to a nulliparous and a lower-parity multiparous control subject, matched for age, hospital, and year of delivery. Cervical examination data were graphed retrospectively from the time of full dilatation. Curves were compared by pairwise likelihood ratio tests, by using a random effects model to adjust for obstetric interventions, with significance set at P <.05. RESULTS Pregnancies in 1095 GMs, 1174 lower-parity multiparous women, and 908 nulliparous women were studied. GMs exhibit a longer initial phase of labor than either nulliparous women or lower-parity multiparous women, begin to dilate rapidly at a greater dilatation than nulliparous women, and experience acceleration of labor at a rate no faster than lower-parity multiparous women. The average labor curve of GMs resembles that of nulliparous women before dilatation of 4 cm is attained, then transitions to the typical curve of the lower-parity multiparous women until dilatation of 6 cm is attained and thereafter is indistinguishable from that of the lower-parity multiparous women (P <.001). CONCLUSIONS Once parity exceeds 4, progress of labor slows. "Poor progress" beyond dilatation of 4 cm should not be considered abnormal for a GM, because she is likely still in the latent phase until dilatation of 6 cm is attained. Nor should she be expected to progress through her active phase any faster than lower-parity multiparous women.