January 16, 2007
Prenatal Testing for Down Syndrome
Amy Movius, MD
Down syndrome, also known as Trisomy 21, is a common genetic disorder caused by the presence of an extra chromosome. People normally have 2 of each type of chromosome (1 from each parent), but individuals with Down syndrome have 3 copies of chromosome 21, hence its other name, Trisomy 21.
People with Down syndrome have mental retardation and high risk of multiple other medical problems. Notably almost half have heart disease, which often required open heart surgery to repair. Intestinal problems, visual trouble, hearing loss, spinal problems, and increased risk of cancer are all other possible challenges these people face. They are also known for having delightful, outgoing personalities, being affectionate and loving music.
The chance of having a baby with Down syndrome increases with maternal age, and so genetic counseling and prenatal testing with amniocentesis or chorionic villus sampling (CVS) has been routinely offered to pregnant women 35 years of age or older. The risk is 1/365 for a 35 year old versus 1/1300 for a 25 year old vs 1/30 for a 45 year old mother. Both amniocentesis and CVS are considered “invasive procedures” and have a small risk of causing pregnancy loss, which is why they have been offered only to“older” pregnant women up to now.
The American College of Obstetrics and Gynocology(ACOG) has just issued new guidelines recommending that all pregnant women be offered less invasive screening to determine their risk for having a Down syndrome baby. These consist of a blood test and an ultrasound in the first trimester with measurement of the nuchal translucency (NT), or thickness at the back of the neck of the fetus. These ultrasounds should be done by providers who have specific training to achieve optimal NT measurements. If these screening tests show an increased risk of having a baby with Down syndrome, these women would be offered genetic counseling and option of CVS or mniocentesis, regardless of age.
Having a baby with Down syndrome has unique challenges; consideration of this possibility by a pregnant woman is avery personal thing. These new recommendations offer high quality screening to all pregnant women and further testing to those found to have an increased risk, with the goal of providing these women with the best information possible in the face of potentially very difficult circumstances.
1. Practice Bulletin #77, “Screening for Fetal Chromosomal Abnormalities”. Obstetrics& Gynecology, January 2007.