ABSTRACT
BACKGROUND: Ambulatory uterine activity monitoring in high-risk women continues despite the results from randomized trials indicating no relationship between monitoring and actual reduction of preterm delivery. The value of uterine contraction frequency as a predictor of preterm delivery, however, remains unclear.
POPULATION STUDIED: A total of 2205 women with a singleton gestation of longer than 22 weeks were screened; 454 met eligibility criteria. Data from 306 women were analyzed, including 254 high-risk women with either a history of preterm delivery (between 20 and 36 weeks’) or bleeding in the 2nd trimester of the current pregnancy, and 52 low-risk women. Exclusion criteria included previous or scheduled use of an ambulatory contraction monitor, use of tocolytic therapy, scheduled cerclage, placenta previa, major fetal anomalies, or no home phone. The mean age of participants was 26.2 years, with a mean parity of 1.8. The majority of participants were black (60%), with at least 12 years of education (74%). Many participants smoked (26%).
STUDY DESIGN AND VALIDITY: The authors used an observational study to determine whether the frequency of contractions could predict spontaneous preterm delivery at less than 35 weeks. Contractions were monitored for at least 30 minutes, twice a day (daytime and nighttime) on 2 or more days per week until 28 weeks, then 4 times per week. Two trained nurses, masked to risk status, analyzed monitor recordings. Contractions were defined as deflections from a clear baseline, with a rounded peak lasting 40 seconds to 120 seconds. Cervical examinations were performed every 2 to 3 weeks, beginning at 22 weeks, up to 6 times, depending on length of gestation. Data collected included cervicovaginal fluid for fetal fibronectin analysis, cervical length by transvaginal ultrasound, and assessment of Bishop score. Assessment of contraction recordings was validated by repeat audits during which samples were re-analyzed. Interpretation discordance occurred in 14% to 28% of recordings, but discrepancies were not greater than 1 contraction per hour.
OUTCOMES MEASURED: The primary outcome was the ability of uterine contraction frequency (daytime and nighttime) to predict spontaneous preterm delivery. In addition, fetal fibronectin, cervical length, and a Bishop score higher than 4 were studied as possible predictors at these same gestational ages.
RESULTS: There was no difference in frequency of contractions between the high-risk and low-risk group and therefore all data were pooled for analysis. The maximal frequency of contractions was inconsistently related to preterm delivery, with the largest association found for nighttime contractions at 27 to 28 weeks (odds ratio [OR] = 1.2; 95% CI, 1.1-1.4). Logistic regression revealed a consistent relationship between ultrasound cervical length and preterm delivery across all gestational age groupings, with statistically significant ORs ranging from 4.0 at 27 to 28 weeks to 7.5 at 31 to 33 weeks. The sensitivity for maximal daytime and nighttime contraction frequency was low, ranging from less than 10% at 22 to 24 weeks to 28% at 27 to 28 weeks and 31 to 33 weeks. Positive PPVs were correspondingly low, with none higher than 25%. Although the sensitivities for fetal fibronectin, ultrasound cervical length assessment, and Bishop scoring were generally somewhat higher (ranging from a low of 19% for fetal fibronectin at 22 to 24 weeks to a high of 82% for cervical length at 31 to 33 weeks) the corresponding PPVs were also low (range = 15% to 37%).
Uterine activity monitoring in asymptomatic high- and low-risk women is inadequate for predicting preterm birth. A recent systematic review of preterm labor management found home uterine activity monitoring by itself ineffective in preventing preterm birth.1 In the current study, contraction frequency monitoring has very poor sensitivity and a low positive predictive value (PPV) for spontaneous preterm delivery before 35 weeks’ gestation. Other commonly used screening tests, such as fetal fibronectin, cervical length assessment, and Bishop scoring, also generally have poor sensitivities and PPVs. The usefulness of any of these tests lies in the reassurance provided by a negative test result, as nearly all of them have negative predictive values of greater than 90%. Understanding, preventing, and treating known causes appears to offer the best current approach to reducing prematurity and its sequellae.