Testing Sooner for Down Syndrome Helps
Study: 1st Trimester Beats 2nd Trimester; Combo Test Also Good
Nov. 9, 2005 -- New research shows that the first trimester of pregnancy may
be the better time to screen for Down syndrome.
That's compared with testing in the second trimester that was performed
between 15 through 18 weeks. The types of testing differed between the two
trimesters. None involved amniocentesis, a more invasive test that samples
fluid from the womb.
Combining results from tests done from each trimester also works well,
researchers report in The New England Journal of Medicine.
About Down Syndrome
The National Institute of Child Health &amp; Human Development provides this
background information on Down syndrome:
- The most frequent genetic cause of mild-to-moderate mental retardation and
associated medical problems
- Occurs in 1 out of 800 live births, in all races and economic groups
- A chromosomal disorder that in most cases is due to an additional third
copy of chromosome 21, or "trisomy 21"
- More likely in babies born to older women
When to Test
The new study was done by doctors including Fergal Malone, MD, of New York's
Columbia University College of Physicians and Surgeons.
Malone's team tracked more than 38,000 women who were screened in their
first trimester of pregnancy. The tests covered several clues about the baby's
likelihood of having Down syndrome.
Each woman was carrying only one baby. Maternal age was taken into account.
A total of 117 women were found to be carrying a fetus with Down syndrome.
The researchers also tested most women during the second trimester of
Testing at 11 weeks of gestation was better than second-trimester screening,
write the researchers. However, testing done later at 13 weeks had similar
results to the second-trimester screening.
Testing both during the first and second trimesters also worked well at
detecting Down syndrome, note the researchers.
They call first-trimester screening "highly effective."
However, they add that combining measurements from first- and
second-trimester tests yielded higher detection rates and lower false-positive
Of course, the combination test meant waiting longer for results.
Options for Patients
Women and doctors may want to weigh the possibility of earlier diagnosis
with lower false-positive rates and greater detection from combination tests,
write Malone and colleagues.
They note that second-trimester testing has been the current standard of
First-trimester screening is "practical" and "clearly superior
to screening in the second trimester," Joe Leigh Simpson, MD, writes in a
Simpson is on staff at Baylor College of Medicine's department of obstetrics
and gynecology. He also works in Baylor's molecular and human genetics
"Pregnant women will now expect the option of first-trimester
screening," Simpson writes. "If not available, it is prudent to permit
a patient to pursue it elsewhere."