ENDOVAGINAL ULTRASONOGRAPHIC MEASUREMENT OF EARLY EMBRYONIC SIZE AS AMEANS OF ASSESSING GESTATIONAL-AGE

Crown-rump length has consistently been found to be the most accurate method of determining gestational age in the first trimester. The orig inal regression curve established by Robinson in 1973 with static arm scanners remains the one most widely employed. New endovaginal ultraso nographic probes afford a degree of detail that allows embryonic struc tures to be seen as soon as they are distinct from the yolk sac. Previ ously, measurements of very early embryonic structures have mistakenly been labeled crown-rump lengths. There has been widespread use of nom ograms constructed from regression curves, where the bulk of the data were derived from small fetuses and then such curves extrapolated back to embryos of very small size. The purpose of this study was to estab lish a nomogram for gestational age assessment by measuring early embr yos prior to the development of a ''crown'' or ''rump.'' This present study consisted of 143 patients. To be included they had to have had n o history of any prior bleeding, and all were delivered of singleton i nfants within 2 weeks of their estimated delivery date by last menstru al period. All had a single early embryonic size measurement between 1 and 25 mm using high-frequency endovaginal probes. Regression analysi s revealed a linear equation of Gestational age (days) = early embryon ic size (mm) + 42 with a correlation coefficient r = 0.87; 95% confide nce limit = +/- 3 days. We conclude that using high-frequency vaginal ultrasonographic probes and having a better understanding of embryonic anatomic stages allow for the construction of a nomogram of gestation al age derived from measurements of early embryonic size prior to deve lopment of a crown-rump length.