The first large scale study of its kind shows that effective screening for Down syndrome is possible before the 14th week of pregnancy--earlier than previous screening regimens have permitted, according to a study appearing in the April 2 New England Journal of Medicine. Currently, screening for Down syndrome is conducted between the 14th and 16th week of pregnancy.
The study, led by James E. Haddow, MD, of the Foundation for Blood Research in Scarborough, Maine, was funded by the National Institute of Child Health and Human Development.
The screening technique makes use of two substances, or markers, for Down syndrome that tend to appear in abnormal amounts in the blood of women carrying fetuses with Down syndrome. Down syndrome, also sometimes referred to as Trisomy 21, is a genetic abnormality resulting from either an extra copy of chromosome 21 or from an extra portion of the long arm of the chromosome. The risk of the condition increases sharply with maternal age. For women under age 30, the risk of conceiving a fetus with Down syndrome is less than 1 in 1,000. By age 35, the risk is 1 in 400, by age 40, 1 in 105, and by age 49, 1 in 12.
Currently, the most effective way to diagnose fetuses with Down syndrome between the 9th and 12th week of pregnancy (during the first trimester) is with a procedure known as chorionic villus sampling (CVS). In this procedure, a needle is inserted into the uterus, either through the abdomen or the cervix, and a sample is extracted from the chorionic villi, the tissue which will later become the placenta. Dr. Haddow explained that this sampling technique, however, carries a one to two percent risk of miscarriages. He added that a reliable test that did not involve removing fetal tissue samples from the uterus would avoid the attendant risk of miscarriage.
Amniocentesis, a technique similar to chorionic villus sampling, is also
occasionally used to diagnose Down
Contact: Bob Bock
NIH/National Institute of Child Health and Human Development