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.

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About Down Syndrome

The National Institute of Child Health & 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

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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 pregnancy.

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.

Combination Tests

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 rates.

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 care.

First-trimester screening is “practical” and “clearly superior to screening in the second trimester,” Joe Leigh Simpson, MD, writes in a journal editorial.

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 department.

“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.”

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By Miranda Hitti, reviewed by Louise Chang, MD

SOURCES: Malone, F. The New England Journal of Medicine, Nov. 10, 2005; vol 353: pp 2001-2011. National Institute of Child Health & Human Development: “Facts About Down Syndrome.” Simpson, J. The New England Journal of Medicine, Nov. 10, 2005; vol 353: pp 2068-2070.