Context Second-trimester prenatal ultrasound is widely used in an attempt to
detect Down syndrome in fetuses, but the accuracy of this method is unknown.
Objective To determine the accuracy of second-trimester ultrasound in detecting
Down syndrome in fetuses.
Data Sources English-language articles published between 1980 and February 1999 identified
through MEDLINE and manual searches.
Study Selection Studies were included if they recorded second-trimester findings of
ultrasonographic markers, chromosomal abnormalities, and clinical outcomes
for a well-described sample of women. A total of 56 articles describing 1930
fetuses with Down syndrome and 130 365 unaffected fetuses were included.
Data Extraction Articles were independently reviewed, selected, and abstracted by 2
reviewers. Discrepancies in data abstraction were resolved by consensus with
a third reviewer. Overall estimates of sensitivity, specificity, and positive
and negative likelihood ratios were calculated for the following markers:
choroid plexus cyst, thickened nuchal fold, echogenic intracardiac focus,
echogenic bowel, renal pyelectasis, and humeral and femoral shortening. Results
were stratified by whether markers were identified in isolation or in conjunction
with fetal structural malformations.
Data Synthesis When ultrasonographic markers were observed without associated fetal
structural malformations, sensitivity for each was low (range, 1%-16%), and
most fetuses with such markers had normal outcomes. A thickened nuchal fold
was the most accurate marker for discriminating between unaffected and affected
fetuses and was associated with an approximately 17-fold increased risk of
Down syndrome. If a thickened nuchal fold is used to screen for Down syndrome,
15 893 average-risk women or 6818 high-risk women would need to be screened
for each case of Down syndrome identified. For each of the other 6 markers,
when observed without associated structural malformations, the marker had
marginal impact on the risk of Down syndrome. Because the markers were detected
in only a small number of affected fetuses, the likelihood of Down syndrome
did not decrease substantially after normal examination findings (none of
the negative likelihood ratios were significant).
Conclusions A thickened nuchal fold in the second trimester may be useful in distinguishing
unaffected fetuses from those with Down syndrome, but the overall sensitivity
of this finding is too low for it to be a practical screening test for Down
syndrome. When observed without associated structural malformations, the remaining
ultrasonographic markers could not discriminate well between unaffected fetuses
and those with Down syndrome. Using these markers as a basis for deciding
to offer amniocentesis will result in more fetal losses than cases of Down
syndrome detected, and will lead to a decrease in the prenatal detection of
fetuses with Down syndrome.