Journal of Ultrasound in Medicine, Vol 20, Issue 7 775-790, Copyright © 2001 by American Institute of Ultrasound in Medicine
Sonographic screening for fetal aneuploidy: first trimester
V. L. Souter and D. A. Nyberg
Center for Perinatal Studies, University of Washington Medical Center, Seattle, USA.
OBJECTIVES: Screening for fetal aneuploidy is now possible during the first
trimester using sonographic and biochemical markers. The aim of this review
was to summarize the efficacy and use of nuchal translucency in screening
for fetal aneuploidy, especially fetal Down syndrome, and other anomalies.
METHODS: We reviewed available literature regarding first-trimester
screening. This includes more than 16 studies of nuchal translucency as a
marker for fetal aneuploidy published since 1995. RESULTS: Although early
studies showed wide variation in detection of fetal Down syndrome when
using nuchal translucency, more recent studies showed sensitivities of
approximately 70% to 80%, for a 5% false-positive rate. Increased nuchal
translucency has also been found to be a marker for other aneuploidies,
including trisomy 18, trisomy 13, and Turner syndrome. Maternal serum
biochemical screening can be used as a test for aneuploidy during the first
trimester The 2 maternal serum markers that appear to be most useful in the
late first trimester are the free beta subunit of human chorionic
gonadotropin and pregnancy-associated plasma protein A. Together with
maternal age, these markers yield a detection rate for trisomy 21 of
approximately 60%, for a 5% false-positive rate. Because sonographic and
biochemical markers appear to be largely independent, their combined risk
results in improved detection rates compared with either method alone. As a
result, the combination of nuchal translucency, biochemical markers, and
maternal age has achieved a detection rate of approximately 85%, for a 5%
false-positive level for detection of trisomy 21. A newly proposed
"integrated" approach using a panel of first- and second-trimester markers
suggests that further improvement in the screening performance is possible.
A number of questions regarding first-trimester screening remain. We
address some of these questions: is first-trimester screening more
effective than second-trimester screening? How to account for intrauterine
lethality? Is earlier diagnosis important, and will it be accepted by
patients? Is first-trimester screening cost-effective? How should
first-trimester screening be interpreted with second-trimester tests?
CONCLUSIONS: Despite encouraging data and general enthusiasm for
first-trimester screening for fetal Down syndrome and other aneuploidies, a
number of questions remain about its implementation in the United States.
Multicenter studies currently under way should help answer some of these
questions.
