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

Abstract

To assess the feasibility of logistic regression analysis for determining the gestational ages at which detection of early pregnancy landmarks first can be observed. Retrospective analysis. University-based tertiary care clinic. Eighty-two women with viable singleton pregnancies in whom ovulation had been achieved by an injection of hCG. Two hundred fifteen transvaginal sonographic scans. Logistic regression was used to estimate the probability of detection of sonographic findings as a function of gestational age. 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. 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.