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Journal of Ultrasound in Medicine, Vol 13, Issue 1 27-31, Copyright © 1994 by American Institute of Ultrasound in Medicine
Endovaginal ultrasonographic measurement of early embryonic size as a means of assessing gestational age
S. R. Goldstein and R. Wolfson
Department of Obstetrics and Gynecology, New York University School of Medicine, New York, USA.
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.
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