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.