Relationship of measured external tocodynamometry with measured internal uterine activity

The use of home uterine activity monitoring for patients at high risk for preterm labor and delivery has become common in clinical perinatology. The ability of the monitoring devices to detect accurately uterine contractions in early pregnancy has not previously been reported. Ten women in labor between 20 and 35 weeks' gestation underwent simultaneous monitoring of uterine activity with a guard-ring tocodynamometer and an intrauterine pressure monitor. When compared with internal monitoring, the external monitor detected 90.8% of uterine contractions with a specificity for uterine quiescence of 98.1%. The predictive value of external monitoring was 97.3% for detecting uterine contractions and 93.6% for recording the absence of uterine contractions. The contractions detected externally were similar in duration: mean 63.7 +/- 23.0 seconds for internal monitoring and 62.2 +/- 22.6 seconds for external monitoring (p greater than 0.05). The intensity of contractions detected externally was less than internally measured contractions, mean difference, 19.7 +/- 15.9 mmHg (p less than 0.001). External tocodynamometry using this guarding tocodynamometer reliably distinguishes between uterine contractions and uterine quiescence in preterm pregnancies but does not adequately measure the intensity of contractions.