Objective: To determine the probability of detecting the gestational sac of a normal intrauterine pregnancy by vaginal ultrasound at different 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 pregnancies who conceived spontaneously or after ovulation induction.
Interventions: Vaginal ultrasound and serum hCG determinations were performed between 20 and 30 days after conception. The timing of the tests was 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 loge (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 multiple gestations were associated with higher serum hCG concentrations than singleton pregnancies of the same gestational age.
Conclusion: If it is known, gestational age rather than the serum hCG concentration should be used to determine whether the gestational sac of an intrauterine pregnancy should be detectable by vaginal ultrasound. Failure to image a gestational sac > or = 24 days after conception is presumptive evidence of an ectopic pregnancy. Reliance on serum hCG rather than gestational age may lead to an erroneous diagnosis of ectopic pregnancy in women with multiple pregnancies.