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 Uterine contractions asymptomatic pregnant women with and without a short cervix (<25 mm) on transvaginal ultra ultrasound sound (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 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 greater than or equal to25 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 : 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 greater than or equal to4 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 greater than or equal to25 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 greater than or equal to4 per hour, compared to <4 per hour. (C) 2004 Elsevier Inc. All rights reserved.