Background: Recognizing the onset of labor can be problematic. Traditionally, pregnant women are advised that experiencing uterine contractions every five minutes for an hour (i.e., 12 or more contractions per hour) indicates the onset of labor; however, no objective evidence appears to support this idea. Pates and colleagues conducted a prospective study to determine if 12 contractions per hour is an indicator of labor.

The Study: The study included all eligible pregnant women presenting to the delivery triage unit of a university hospital in Texas during a three-month period. Criteria for participation included 36 or more weeks' gestation, cervical dilation of 3.0 cm or less, intact membranes, and no known medical or obstetric complications. All study participants were assessed on admission, including cervical examination. Those who had a dilation of 4.0 cm or more were admitted to the delivery unit. The remaining study participants were observed using external fetal heart monitoring for at least one hour.

Data from tocodynamometry recordings were analyzed to define the presence and frequency of contractions in each patient. A contraction was defined as a distinct elevation and return to baseline lasting at least 30 seconds. Data on uterine contractions were correlated with information on time and method of delivery, obstetric complications, and demographic data.

Results: Contraction frequencies of 12 or more per hour were documented in 352 (46 percent) of the 768 eligible participants. Women with 12 or more contractions per hour were significantly more likely to be in active labor on admission compared with women experiencing less than 12 contractions per hour (56 versus 28 percent, respectively). Overall, labor was significantly more likely to be diagnosed within 24 hours of assessment in the women who had 12 or more contractions per hour (268 women [76 percent]) compared with the women who had fewer contractions (216 women [52 percent]). The positive predictive value for diagnosis of labor with 12 or more contractions per hour was 56 percent on admission and 76 percent within 24 hours. In multivariate analysis, having 12 or more contractions per hour remained statistically significant after adjustment for all important obstetric variables. Obstetric and neonatal outcomes were not affected by monitoring the frequency of uterine contractions on admission.

Conclusion: The authors conclude that advising women to come to the hospital when experiencing contractions five minutes apart is supported as a valid warning of onset of labor. They stress that women could be in active labor with lower rates of contractions but advise that in the absence of other indicators (e.g., ruptured membranes), 12 contractions or more per hour is a good indicator of imminent labor.