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There is a paucity of data on how much uterine activity is excessive when measured against clinical outcomes. Our purpose was to determine if uterine tachysystole as defined by the American Congress of Obstetricians and Gynecologists, 6 or more contractions per 10 minutes, is associated with adverse fetal outcomes.
This is a prospective cohort study of 584 women undergoing induction of labor with 100 micrograms of oral misoprostol. Fetal heart rate tracings were analyzed for contractions per 10 minutes during the initial 4 hours after misoprostol administration. Patients were divided into 4 groups: ≤4, 5, 6, ≥7 contractions per 10 minutes. Infant condition at birth was assessed using 5- minute Apgar ≤3, umbilical artery blood pH ≤7.1, intubation in the delivery room, neonatal seizures, NICU admission, or perinatal death. Cesarean delivery for FHR indications was also analyzed.
Infant outcome and cesarean birth for FHR indications showed no associations with the number of contractions per 10 minutes (Table). However, 6 or more contractions in 10 minutes were significantly associated with FHR decelerations (p=<0.001); this was true for variable, late, and prolonged decelerations (p=<0.001, p=0.02, p=0.017 respectively). Analysis was also performed using the maximum number of contractions per 30 minutes and the results were similar to those for 10- minute intervals.
Uterine activity quantified as the maximum number of contractions per ten minutes or per 30 minutes was not found to be associated with cesarean delivery or any measure of infant condition at birth that could serve to define excessive uterine contractions. However, 6 or more contractions per ten minutes was associated with increased fetal heart rate decelerations. We conclude that the current recommended definition of uterine tachysystole by the American Congress of Obstetricians and Gynecologists as 6 or more contractions per ten minutes is an appropriate definition.