Journal of Ultrasound in Medicine, Vol 12, Issue 11 647-653, Copyright © 1993 by American Institute of Ultrasound in Medicine
Improved prediction of gestational age in the late third trimester
P. M. Doubilet and C. B. Benson
Department of Radiology, Harvard Medical School, Brigham and Women's Hospital, Boston, Massachusetts 02115.
A variety of formulas have been published for predicting GA in the second
and third trimesters. We assessed these formulas for the presence of
systematic errors. Our study population consisted of 1036 obstetrical
sonograms at 14 to 42 weeks' gestation in patients with highly accurate
dating based on a prior first trimester scan. Most formulas had mean errors
(mean value of the difference between predicted and true GA) of no more
than 1 week between 14 and 38 weeks' gestation. At 38 to 42 weeks, however,
all formulas had large systematic underestimation biases, with mean
negative errors in the range of 1.6 to 3 weeks. By regressing ln(GA)
against one or more measurements, we developed new formulas that eliminate
this underestimation bias, with mean errors of no more than 0.6 week
throughout the entire 14 to 42 week period. As with current formulas,
however, our formulas have wide 95% confidence ranges of +/- 3 weeks or
greater at 38 to 42 weeks as a result of inherent biological variability.
We recommend the use of these formulas in clinical practice, as they may
prevent errors in diagnosis late in pregnancy.
