THE DISCRIMINATORY HUMAN CHORIONIC-GONADOTROPIN ZONE FOR ENDOVAGINAL SONOGRAPHY - A PROSPECTIVE, RANDOMIZED STUDY

Objective: To determine the probability of detecting the gestational s ac of a normal intrauterine pregnancy by vaginal ultrasound at differe nt gestational ages and serum hCG concentrations. Design: Prospective, randomized study. Setting: Pregnant human volunteers in a university- based clinical research environment. Patients: Women with viable pregn ancies who conceived spontaneously or after ovulation induction. Inter ventions: Vaginal ultrasound and serum hCG determinations were perform ed between 20 and 30 days after conception. The timing of the tests wa s determined randomly. Main Outcome Measure: Detection of gestational sac. Results: The probability of detecting a gestational sac increased significantly with both gestational age and serum hCG concentration, but the regression on gestational age fitted the data much better than the regression on log(e)(hCG). The probability of detecting a sac was similar in multiple and singleton pregnancies of the same gestational age but, for a given hCG concentration, the probability of detecting a sac was lower for multiple than for singleton pregnancies because mu ltiple gestations were associated with higher serum hCG concentrations than singleton pregnancies of the same gestational age. Conclusion: I f it is known, gestational age rather than the serum hCG concentration should be used to determine whether the gestational sac of an intraut erine pregnancy should be detectable by vaginal ultrasound. Failure to image a gestational sac greater than or equal to 24 days after concep tion is presumptive evidence of an ectopic pregnancy. Reliance on seru m hCG rather than gestational age may lead to an erroneous diagnosis o f ectopic pregnancy in women with multiple pregnancies.