by Christina Churchill and Susan Dudley, PhD
July 2006
Down's syndrome is the most common cause of developmental disability.
Children with Down's syndrome typically have mild to moderate
mental retardation, along with several distinctive physical characteristics
and medical problems.
Approximately one out of every 800 babies has Down's syndrome.
Because of a genetic mistake that occurs at the very earliest
stages of development, they have an extra copy of chromosome-21
in their cells.
The likelihood that a baby will be born with Down's syndrome increases
with maternal age. While fewer than 1 in 1000 children born to
mothers who are under 30 have Down's syndrome, the odds increase
to 1 in 105 among 40 year old mothers, and to 1 in 20 for women
pregnant at age 45. For this reason, many older pregnant women
are interested in getting prenatal Down's screening.
Test Variables
There are several prenatal screening and diagnostic tests for
Down's syndrome. There are advantages and disadvantages to each
of them, because they vary in terms of
Is Earlier Screening Practical?
In the United States, most Down's screening is done in the second
trimester of pregnancy. By the time the test results are available
for prospective parents to learn whether their baby has tested
positive for Down's syndrome, the pregnancy is well advanced.
Most people agree that there would be many reasons to favor
first-trimester screening if parents could be assured that the
results will be accurate.
A recent study suggests that first-trimester screening can,
in fact, be very effective when several tests are combined to
calculate risk of Down's syndrome. Researchers tested more than
38,000 pregnant women in order to determine the most accurate
results with the smallest potential for harm at the earliest
point in pregnancy. They compared first-trimester tests, second-trimester
tests, and test that spanned both first and second trimesters.
The researchers concluded that when the mother's age is taken
into account, 87% of Down's cases can be detected by combining
a specialized ultrasound test and a simple maternal blood test
at 11 weeks of pregnancy.1 Experts believe that this
will be an important breakthrough for parents whose pregnancies
are at risk because of maternal age or other factors. It means
that only 13 babies with Down's syndrome would be missed out
of every 80,000 pregnant women screened in the general population.
The researchers also found that in situations where the specialized
ultrasound test is inconclusive or not available, doctors can
combine the 11-week maternal blood test with a second-trimester
maternal blood test2 taken at 15-18 weeks to detect
96% of Down's cases.
Earlier Screening and Later Testing
Earlier, non-invasive screening tests can provide many advantages.
Most of the time the results will be negative, and the parents'
anxiety will be put to rest. And, the earlier that parents learn
that Down's is probable, the more time they have to weigh their
options and make difficult decisions.3
Additional diagnostic testing can be performed on babies who
screen positive for Down's syndrome, using amniocentesis or
chorionic villus sampling (CVS).4 Since both of those tests have
a risk of causing miscarriage, it is very important that the
initial screening be as accurate as possible. First-trimester
screening will mean that parents can be better informed in making
decisions about whether they want to risk having these more
invasive but more accurate tests.
For parents who choose to continue their pregnancies after a
Down's diagnosis, first-trimester results give them more time
to become educated and prepared for raising a child with Down's
syndrome. Those who decide to terminate their pregnancies could
do so at an earlier point, when there are fewer risks to the
woman's health.
For more information on Down's Syndrome:
http://www.nichd.nih.gov/publications/pubs/downsyndrome/down.htm
Source: FD Malone, and others. First-Trimester or Second-Trimester
Screening, or Both, for Down's Syndrome.
The New England Journal
of Medicine, November 10, 2005, pages 2001-2011.
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1. They used a specialized 20-minute sonogram measuring nuchal
translucency (the thickness of the skin on the back of the fetus's
neck) along with a maternal blood test measuring pregnancy associated
plasma protein-A (PAPP-A) and the free beta subunit of human
chorionic gonadotropin (fßhCG).
2. They measured a-fetoprotein, total hCG, unconjugated estriol
(uE3), and Inhibin-A.
3. Accuracy of the tests in this study was calculated using
the mathematical assumption that there is also a 5 in 100 chance
that the screening test will indicate Down's syndrome when it
is not actually present.
4. Amniocentesis and chorionic villus sampling (CVS) are more
accurate than the other tests.
