Objective: Our purpose was to determine whether the risk for postdatism can be reduced by serial membrane sweeping in women with an unfavorable cervix at 39 weeks’ gestation and a negative fetal fibronectin test result. Study Design: Women with uncomplicated pregnancies, who were candidates for a vaginal delivery with an unfavorable cervix at 39 weeks’ gestation and a negative fetal fibronectin test result were asked to participate in this investigation. Patients were chosen at random and assigned to a group for membrane sweeping every 3 days or to a control group who received gentle examinations every 3 days. Results: Sixty-five women were selected at random for serial membrane sweeping (n = 33) or for the control group (n = 32). Although gestational age and Bishop score at study entry were similar, the gestational age on admission for delivery was earlier in the membrane sweeping group (39.9 ± 0.3) versus the control group (41.5 ± 0.6, P < .0001). The Bishop score on admission to labor and delivery was greater (8.8 ± 2.1) in the membrane sweeping group than in the control group (6.2 ± 2.7, P < .0001). The number of women admitted for labor inductions at 42 weeks’ gestation was 18 of 32 (56%) in the control group versus none (0 of 24) in the membrane-sweeping group (P < .0001). Conclusions: Women with an unfavorable cervix at 39 weeks’ gestation and a negative fetal fibronectin test result are at risk for not being delivered by 41 completed weeks and thus may require postdates induction or antenatal testing. Serial membrane sweeping significantly reduces the risk of postdatism and induction of labor. (Am J Obstet Gynecol 1998;179:890-4.)