Identification of early pregnancy landmarks by transvaginal sonography: analysis by logistic regression.

OBJECTIVE: To assess the feasibility of logistic regression analysis for determining the gestational ages at which detection of early pregnancy landmarks first can be observed. DESIGN: Retrospective analysis. SETTING: University-based tertiary care clinic. PATIENT(S): Eighty-two women with viable singleton pregnancies in whom ovulation had been achieved by an injection of hCG. INTERVENTION(S): Two hundred fifteen transvaginal sonographic scans. MAIN OUTCOME MEASURE(S): Logistic regression was used to estimate the probability of detection of sonographic findings as a function of gestational age. RESULT(S): We found that the likelihood of visualization of a gestational sac or fetal heart motion could be represented accurately by logistic equations. Gestational age at which there was 95% probability of visualization was 35.5 days for the gestational sac and 44.5 days for fetal cardiac activity. The probability of detecting fetal cardiac activity was 95% when the mean gestational sac diameter was 1.6 cm and was 99% at 1.9 cm. CONCLUSION(S): The sonographic appearances of developmental landmarks in early pregnancy occurs within well-defined gestational time periods, and the probabilities for visualization can be closely approximated using a logistic model. Our results suggest that the number of sonographic examinations required to document infertility treatment success can be minimized by surveillance at standardized gestational ages.