Cervix length and relaxin as predictors of preterm birth

Measurement of mid-gestation cervix length has become a common screening tool for preterm birth. Our study was designed to assess the value of cervix length and serum relaxin in the prediction of spontaneous preterm birth at < 35 and < 37 weeks' gestation in a general obstetric population.

A prospective cohort of women with a singleton pregnancy had blood collected at 24 and 28 weeks' gestation for determination of serum relaxin immediately before transvaginal ultrasound measurement of cervical length. Patients and referring physicians were blinded to cervix length and relaxin levels to preclude changes in management. The primary outcomes were spontaneous delivery at < 35 and < 37 weeks' gestation.

A total of 1004 women entered the study. Delivery data were not available for 20 women who delivered elsewhere. Twenty women were excluded because preterm delivery was undertaken because of pregnancy abnormalities. Of the 964 women with known gestational age at delivery, 46 (4.8%) delivered at < 37 weeks and 16 (1.7%) at < 35 weeks' gestation. Mean cervix length at 28 weeks (36.7 +/- 7.3 mm) was significantly shorter than at 24 weeks (37.8 +/- 7.1 mm) (P < 0.001). Cervix length at 24 and 28 weeks' gestation was equally predictive of preterm birth. A cervix length of < or = 30 mm at 28 weeks had a sensitivity of 57.1%, a specificity of 81.8%, and a positive predictive value of 4.5% for birth at < 35 weeks. Serum relaxin levels were not correlated with cervix length at either 24 or 28 weeks. Serum relaxin at 24 and 28 weeks' gestation was not associated with preterm birth before or after controlling for patient characteristics and cervix length.

Serum relaxin levels at 24 and 28 weeks' gestation are not associated with preterm birth. Although cervix length is associated with preterm birth, its positive predictive value is low. Given the lack of proven therapies for those at risk, cervix length does not appear to be a useful screening tool for preterm delivery in the general obstetric population.