Frequency of uterine contractions in asymptomatic pregnant women with or without a short cervix on transvaginal ultrasound scan.

OBJECTIVE: The purpose of this study was to compare the frequency of uterine contractions in asymptomatic pregnant women with and without a short cervix (< 25 mm) on transvaginal ultrasound (TVU) and to determine the additive risk of contractions on the risk of preterm birth. STUDY DESIGN: The study involved secondary analysis of a blinded observational study of asymptomatic singleton pregnancies who were at high risk for preterm birth and who received both home uterine activity monitoring daily and transvaginal ultrasound of the cervix at 22 to 24 and 27 to 28 weeks of gestation. Thresholds for the maximum frequency of uterine contractions of 4 per hour and transvaginal ultrasound cervical length of 25 mm were used for analysis. Contraction frequency was compared in women with cervical length < 25 mm and > or =25 mm and was correlated with the risk of spontaneous preterm birth at < 35 weeks of gestation. RESULTS: Of the 303 women whose pregnancy was evaluated at 22 to 24 weeks of gestation, the 39 women (13%) with a cervical length of < 25 mm had 1.6 +/- 2.7 versus 1.2 +/- 2.0 contractions per hour in the 264 women (87%) with a cervical length of > or =25 mm (P=.37). At 27 to 28 weeks of gestation (n=295 women), contraction frequency was 3.2 +/- 3.7 versus 2.8 +/- 3.1 contractions per hour in women with a cervical length of < 25 mm (n=59 women; 20%) versus those with a cervical length of > or =25 mm (n=236 women; 80%; P=.34). Among women with a short cervix, the relative risks for spontaneous preterm birth were 2.0 (95% CI, 0.95-4.2) and 2.1 (95% CI, 1.06-4.3) for women with > or =4 contractions per hour compared with women with < 4 contractions per hour at 22 to 24 and 27 to 28 weeks of gestation, respectively. Results were confirmed by logistic regression analysis. CONCLUSION: The frequency of uterine contractions in asymptomatic women was not related significantly to cervical length of < 25 mm versus > or =25 mm. Among women with a cervical length of < 25 mm at 22 to 24 or 27 to 28 weeks of gestation, there was a trend toward a 2-fold increased risk of spontaneous preterm birth when the maximum contraction frequency was > or =4 per hour, compared to < 4 per hour.

Frequency of uterine contractions and the risk of spontaneous preterm delivery.

BACKGROUND: The measurement of the frequency of uterine contractions has not been useful for reducing the rate of preterm delivery in randomized trials. Nonetheless, ambulatory monitoring of contractions continues to be used in clinical practice. METHODS: We assessed the frequency of uterine contractions as a predictor of the risk of spontaneous preterm delivery before 35 weeks of gestation. We enrolled women with singleton pregnancies between 22 and 24 weeks of gestation. The women used a contraction monitor at home to record contraction frequency twice daily on 2 or more days per week from enrollment to delivery or 37 weeks of gestation. RESULTS: We obtained 34,908 hours of successful monitoring recordings from 306 women. Although more contractions were recorded from women who delivered before 35 weeks than from women who delivered at 35 weeks or later, we could identify no threshold frequency that effectively identified women who delivered preterm infants. The sensitivity and positive predictive value of a maximal hourly frequency of contractions of four or more between 4 p.m. and 3:59 a.m. were 9 percent and 25 percent, respectively, at 22 to 24 weeks and 28 percent and 23 percent at 27 to 28 weeks. Other proposed screening tests, such as digital and ultrasound evaluations of the cervix and assays for fetal fibronectin in cervicovaginal secretions, also had low sensitivity and positive predictive value for preterm labor. CONCLUSIONS: Although the likelihood of preterm delivery increases with an increased frequency of uterine contractions, measurement of this frequency is not clinically useful for predicting preterm delivery.