Your health care provider will use your age, race and the results of the quad screen to gauge your risk of carrying a baby who has certain chromosomal conditions or neural tube defects. Other factors — such as your personal or family health history — also might affect your risk.
Quad screen results are given 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 250 to 1 in 300.
Keep in mind that a positive quad screen simply means that levels of some or all of the substances measured in your blood were outside the normal range. This can happen for various reasons, including:
- A miscalculation of how long you've been pregnant
- A multiple pregnancy
- In vitro fertilization
- The presence of other medical conditions, such as diabetes
- Smoking during pregnancy
If your test results are positive, your health care provider might recommend an ultrasound to verify the baby's gestational age and confirm the number of babies. During the ultrasound, your health care provider will also evaluate your baby's growth, study the placenta and identify possible abnormalities.
The quad screen correctly identifies about 80 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 the quad screen only indicates your overall risk of carrying a baby who has certain chromosomal conditions or neural tube defects. 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. If you're at high risk of a chromosomal condition, you might consider a new prenatal screening test that analyzes fetal DNA circulating within your blood. This blood test evaluates 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.
- Targeted ultrasound. If you're at high risk of a neural tube defect, your health care provider might suggest a targeted ultrasound — perhaps accompanied by amniocentesis. Ultrasound isn't an effective screening tool for Down syndrome.
- 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 and isn't useful in detecting neural tube defects, such as spina bifida.
- 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
- Canick JA, et al. Second trimester maternal serum screening for Down syndrome. http://www.uptodate.com/index. Accessed Aug. 27, 2012.
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