The quad screen measures levels of AFP, HCG, estriol and inhibin A in a pregnant woman's blood. Your health care provider will use your age at the estimated time of delivery and the results of the quad screen to evaluate your chance of carrying a baby who has certain chromosomal conditions, neural tube defects or abdominal wall defects.
Quad screen results are given as a probability, such as a 1 in 500 risk of carrying a baby who has Down syndrome. 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. Factors that can affect the substances measured by a quad screen include:
- A miscalculation of your baby's age (how long you've been pregnant)
- Maternal race
- Maternal weight
- A multiple pregnancy
- In vitro fertilization
- 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.
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 chance of carrying a baby who has certain chromosomal conditions or neural tube defects. A decreased chance (negative screen result) doesn't guarantee that your baby won't have one of these conditions. Likewise, an increased chance (positive screen result) doesn't guarantee that your baby will be born with one of these conditions.
Often, positive screen results are an invitation to consider secondary testing. For example:
- Prenatal cell-free DNA screening. This sophisticated blood test examines cell-free DNA from the placenta and the fetus in the maternal bloodstream. This screening evaluates whether your baby is at risk of Down syndrome, extra sequences of chromosome 13 (trisomy 13), extra sequences of chromosome 18 (trisomy 18) or a sex chromosome abnormality, such as Turner syndrome. A normal result might eliminate the need for an invasive prenatal diagnostic test.
- Targeted ultrasound. If you're at high risk of a neural tube defect, your health care provider might suggest a targeted ultrasound. Ultrasound isn't an effective screening tool for Down syndrome.
- Chorionic villus sampling (CVS). This procedure 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. 21, 2015
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