Endovaginal ultrasonographic measurement of early embryonic size as a means 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 original regression curve established by Robinson in 1973 with static arm scanners remains the one most widely employed.  New endovaginal ultrasonographic probes afford a degree of detail that allows embryonic structures to be seen as soon as they are distinct from the yolk sac.  Previously, measurements of very early embryonic structures have mistakenly been labeled crown-rump lengths.  There has been widespread use of nomograms 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 establish a nomogram for gestational age assessment by measuring early embryos prior to the development of a "crown" or "rump."  This present study consisted of 143 patients.  To be included they had to have had no history of any prior bleeding, and all were delivered of singleton infants within 2 weeks of their estimated delivery date by last menstrual period.  All had a single early embryonic size measurement between 1 and 25 mm using high-frequency endovaginal probes.  Regression analysis revealed a linear equation of Gestational age (days) = early embryonic size (mm) + 42 with a correlation coefficient r = 0.87; 95% confidence 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 gestational age derived from measurements of early embryonic size prior to development of a crown-rump length.