Subclassification of indeterminate pelvic ultrasonography: prospective evaluation of the risk of ectopic pregnancy.

STUDY OBJECTIVE: We sought to determine the frequency of ectopic pregnancy among subclasses of indeterminate ultrasonographic examinations.

METHODS: A prospective observational study was performed from January 1, 1995, to August 31, 2000, on consecutive emergency department patients in the first trimester of pregnancy with a chief complaint of abdominal pain or vaginal bleeding and who had an indeterminate transvaginal ultrasonographic examination at the time of the ED visit. Patients were excluded if lost to follow-up. Ultrasonographic examinations were subclassified into 5 groups (ie, empty uterus, nonspecific fluid, echogenic material, abnormal sac, normal sac) on the basis of a previously published classification system. Patients were followed up until the diagnosis of ectopic pregnancy was either confirmed or excluded. The frequencies of ectopic pregnancy, along with 95% confidence intervals (CIs), were calculated for each of the subclasses. The relative risk of ectopic pregnancy was calculated when appropriate.

RESULTS: Seven hundred eighty patients with indeterminate ultrasonographic examinations were identified. One hundred forty-five were lost to follow-up, and therefore, 635 were enrolled. The frequency of ectopic pregnancy for each subclass is as follows: empty uterus, 36 of 259 (13.9%; 95% CI 10.1% to 18.5%); nonspecific fluid, 6 of 127 (4.7%; 95% CI 1.9% to 9.6%); echogenic material, 4 of 93 (4.3%; 95% CI 1.4% to 10.5%); abnormal sac, 0 of 103 (0%; 95% CI 0.0% to 2.9%); and normal sac, 0 of 53 (0%; 95% CI 0.0% to 5.5%). The relative risk of ectopic pregnancy in patients with an empty uterus versus in those without an empty uterus was 5.2 (95% CI 2.6 to 10.2).

CONCLUSION: In our sample, patients with an empty uterus at ultrasonography had the highest frequency of ectopic pregnancy, with a relative risk of ectopic pregnancy 5 times greater than that of the other 4 subclasses.