Your health care provider will use your age and the results of the blood test and ultrasound exam to gauge your risk of carrying a baby who has Down syndrome or Edwards syndrome. Other factors — such as your personal or family health history — also might affect your risk.

First trimester screening results are given as positive or negative and also as a probability, such as a 1 in 5,000 risk of carrying a baby who has Down syndrome. Generally, the test is considered positive if the risk is 1 in 300 or higher.

First trimester screening correctly identifies about 85 percent of women who are carrying a baby who has Down syndrome. About 5 percent of women have a false-positive result, meaning that the test result is positive but the baby doesn't actually have Down syndrome.

When you consider your test results, remember that first trimester screening only indicates your overall risk of carrying a baby who has Down syndrome or Edwards syndrome. A low-risk result doesn't guarantee that your baby won't have one of these conditions. Likewise, a high-risk result doesn't guarantee that your baby will be born with one of these conditions.

Often, positive test results are an invitation to consider secondary testing. For example:

  • Noninvasive prenatal testing. This new blood test analyzes fetal DNA circulating within your blood to gauge your risk of carrying a baby who has Down syndrome, as well as various other chromosomal conditions — perhaps helping you avoid testing that's more invasive.
  • Chorionic villus sampling (CVS). CVS can be used to diagnose chromosomal conditions, such as Down syndrome. During CVS, which is usually done during the first trimester, a sample of tissue from the placenta is removed for testing. CVS poses a slight risk of miscarriage.
  • Amniocentesis. Amniocentesis can be used to diagnose both chromosomal conditions, such as Down syndrome, and neural tube defects, such as spina bifida. During amniocentesis, which is usually done during the second trimester, a sample of amniotic fluid is removed from the uterus for testing. Like CVS, amniocentesis poses a slight risk of miscarriage.

Your health care provider or a genetic counselor will help you understand your test results and what the results mean for your pregnancy.

Oct. 13, 2012