Traditionally, pregnant women 35 and older at time of delivery have been considered at highest risk of giving birth to a baby with Down syndrome and have been urged to get tested.
The new recommendations from the American College of Obstetricians and Gynecologists call for prenatal screening tests for Down syndrome to be offered to all pregnant women.
The new screening tests are far less invasive than older, albeit more definitive, diagnostic tests, such as amniocentesis.
While screening tests can't provide a diagnosis, they do indicate who is at increased risk and should then be checked by amniocentesis or another invasive test, chorionic villus sampling (CVS).
Down syndrome is a common genetic birth defect that affects approximately one in 800 children.
Babies with Down syndrome have an extra chromosome, which causes developmental differences in the brain and body. They can include mental retardation, a characteristic flat facial appearance, serious heart defects, and other medical problems.
Down Syndrome Testing No Longer Based on Age
In creating the revised recommendations, which appear in the January issue of the journal Obstetrics and Gynecology, researchers reviewed studies on ways to screen for Down syndrome developed in the last decade. These screens combine ultrasound examination and blood tests.
The scientists also reviewed the age cutoff of 35 for recommending Down syndrome diagnostic testing.
Genetic defects such as Down syndrome can be diagnosed with the more invasive amniocentesis.
But the test involves inserting a needle to draw a fluid sample from the amniotic sac surrounding the fetus for genetic analysis. It isn't normally done until the second trimester and is associated with a small risk of miscarriage.
The new, noninvasive screening tests can be performed earlier, during the first trimester of pregnancy, giving women more information sooner.
And although the risk of having a baby with Down syndrome does increase with the mother's age, researchers say it is a gradual increase that doesn't jump suddenly at 35.
New Screening Options
Topping the list of recommended first trimester tests is a screening that combines blood tests with an ultrasound exam called a "nuchal translucency test." This exam measures the thickness at the back of the fetus' neck.
A nuchal translucency exam alone, without the blood testing, is not as effective.
A woman determined to be at increased risk of carrying a baby with Down syndrome based on these tests should be offered genetic counseling and the option of either amniocentesis or CVS, which also uses a sample of fetal cells for genetic analysis, according to ACOG.
There are pros and cons to each testing method, ACOG says, and pregnant women should discuss the options with a doctor.
SOURCE: Obstetrics & Gynecology, January 2007; vol 109: pp 217-27.
By Jennifer Warner
Reviewed by Louise Chang