A short cervix in women with preterm labor and intact membranes: A risk factor for microbial invasion of the amniotic cavity

Objective The purpose of this study was to determine whether there was a relationship between sonographic cervical length and the presence of culture-proven microbial invasion of the amniotic cavity in women with preterm labor and intact membranes. Study design Ultrasonography and amniocentesis were performed in 401 patients admitted with preterm labor (22-35 weeks) and cervical dilatation of ≤3 cm, as assessed by digital examination. Cervical length was determined by transvaginal ultrasound at admission. Outcome variables were the presence of microbial invasion of the amniotic cavity (defined as a positive amniotic fluid culture) and the occurrence of preterm delivery before 35 weeks. Contingency tables, χ2 test, receiver-operator characteristic (ROC) curves, and logistic regression were used for statistical analysis. Results The prevalence of microbial invasion of the amniotic cavity was 7% (28/401). Spontaneous preterm delivery (≤35 weeks) occurred in 21.4% (82/384) of patients. ROC curve analysis showed a significant relationship between the frequency of microbial invasion of the amniotic cavity and the length of the uterine cervix (area under the curve: 0.77; P< .005). Patients with a cervical length <15 mm had a higher rate of a positive amniotic fluid culture than patients with a cervical length ≥15 mm (26.3% [15/57] vs. 3.8% [13/344], respectively; P< .05). Moreover, patients with a short cervix (defined as <15 mm) were more likely to deliver spontaneously before 35 weeks, 32 weeks, within 7 days, and within 48 hours of admission (P< .05 for all comparisons). Forty percent of patients (161/401) had a cervical length ≥30 mm. These patients had a very low risk of microbial invasion of the amniotic cavity (1.9% [3/161]), spontaneous delivery ≤35 weeks (4.5% [7/154]), ≤32 weeks (2.6% [2/76]), within 7 days (1.9% [3/154]), and within 48 hours (0% [0/154]) of admission. Conclusion Endovaginal ultrasonographic examination of the uterine cervix in women with preterm labor identifies patients at increased risk for intrauterine infection.