Journal of Ultrasound in Medicine, Vol 14, Issue 4 297-302, Copyright © 1995 by American Institute of Ultrasound in Medicine
Can the presumed risk of autosomal trisomy be decreased in fetuses of older women following a normal sonogram?
A. S. Nadel, B. Bromley, F. D. Frigoletto Jr and B. R. Benacerraf
Department of Obstetrics, Massachusetts General Hospital, Harvard Medical School, Boston, USA.
Previous studies showed that a sonographic scoring system can be used to
identify women under age 35 who are at an increased risk for a fetal
autosomal trisomy. We propose to use the same scoring system to select
women over age 35 who are at a decreased risk of a fetal autosomal trisomy
because the ultrasonogram is normal. We reviewed the sonographic scores of
97 trisomic and 694 normal control fetuses from two previously published
consecutive series designed to identify women at increased risk for an
affected fetus. Using the same scoring index in this study, we calculated
the sensitivity and specificity of a score = 0 zero for identifying fetuses
with autosomal trisomy. We then applied Bayes' theorem to determine the
probability of an autosomal trisomy at various maternal ages, given a
sonographic score of 0. Of the 97 fetuses with an autosomal trisomy, 83 had
a score of > or = 1, for a sensitivity of 86% (95% CI 77-92%). Of the
694 control fetuses with normal karyotype, 606 had a score of 0, for a
specificity of 87% (95% CI 83-89%). A 42 year old woman's probability of an
autosomal trisomy if the sonographic score is 0 becomes equivalent to the
age-specific probability for a 35 year old woman. Using the lower limit of
the CIs for sensitivity and specificity, we calculated that the probability
of having a fetus with an autosomal trisomy falls from 18.8 in 1000 to 5.3
in 1000 for a 40 year old woman with a sonographic score of 0.(ABSTRACT
TRUNCATED AT 250 WORDS)
