All pregnant women, regardless of their age, should be offered screening for Down syndrome in their first trimester, according to new practice guidelines issued today by the American College of Obstetricians and Gynecologists.
Noninvasive screening tests developed during the past decade have made it possible to predict the risk of Down syndrome and certain other chromosomal abnormalities with high accuracy.
Pregnancies identified as being at risk then can be offered further screening with invasive, and more dangerous, tests such as amniocentesis or chorionic villus sampling, according to the guidelines published in the January issue of the journal Obstetrics & Gynecology.
Invasive tests have been offered routinely to women older than 35, who are at the highest risk of bearing affected children. But the risks in older women have been managed so successfully that the largest numbers of Down syndrome children are being born to younger women, according to Dr. Edward McCabe of the Mattel Children’s Hospital at the University of California, Los Angeles.
Down syndrome is caused by the presence of an extra copy of chromosome 21 and is characterized by congenital heart defects and mental retardation.
The risk is low – about one case for every 1,300 births – in young women, then increases sharply to one in 350 births beyond age 35 and one in 25 beyond 45.
The original rationale for screening only older women was that the invasive tests could be dangerous for the fetus, leading to one death in about every 200 examinations. That risk has declined during the past decade, but many women still prefer to avoid it unless there is reason to believe the Down risk is higher, said Dr. Nancy Green, medical director of the March of Dimes, which is focused on preventing birth defects.
Beginning in the 1990s, obstetricians began using assays for three chemicals – alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin – in the mother’s bloodstream during the first trimester. This so-called triple test identifies about 70 percent of Down cases.
A major breakthrough occurred at the end of the decade with the recognition that an accumulation of fluid in the fetus’ neck is associated with a broad range of chromosomal abnormalities, including Down syndrome. This nuchal translucency can be measured by ultrasound.
Major trials in the United States and the United Kingdom have shown that adding ultra sonography for nuchal translucency to the triple test raises the detection rate above 90 percent.
The new guidelines thus call for all women to be offered a combination of blood and nuchal translucency tests during the first trimester. Those who test positive should be offered either amniocentesis or chorionic villus sampling during the second trimester.