quad screening for birth defects

test overview

the quad screening is a blood test that may be done at 15 to 22 weeks of pregnancy. it's used to look for possible problems with your baby. the quad screening measures the amounts of four things in a pregnant woman's blood. they are:

• alpha-fetoprotein (afp). afp is made in the liver of an unborn baby (fetus).
• human chorionic gonadotropin (hcg). this hormone is made by the placenta when a woman becomes pregnant.
• estriol (ue3). this is a form of estrogen that increases during pregnancy. it's made in large amounts by the placenta.
• hormone inhibin a. this hormone is produced by the baby and the placenta.

this test can't show for sure that your baby has a birth defect. you would need a diagnostic test called amniocentesis to find out for sure if there is a problem.

why it is done

the quad screening is done to find out the chance that your baby has certain birth defects, such as down syndrome, spina bifida, or anencephaly.

how it is done

a health professional uses a needle to take a blood sample, usually from the arm.

watch

results

a "positive" result means that there is a higher-than-average chance your baby has a birth defect. if the result is "negative," or normal, it means that your baby probably doesn't have a birth defect. but it doesn't guarantee that you will have a normal pregnancy or baby.

the accuracy of a quad screening test is based on how often the test correctly finds a birth defect.

• it correctly finds neural tube defects, such as spina bifida, in up to 80 out of 100 fetuses who have it. it finds anencephaly in about 95 out of 100 fetuses. the test misses spina bifida in at least 20 out of 100 fetuses who have it. and the test misses anencephaly in about 5 out of 100 fetuses.^{footnote 1}
• it correctly finds down syndrome in 81 out of 100 fetuses who have it. it misses down syndrome in 19 out of 100 fetuses.^{footnote 2}

your doctor may tell you the result of your test as a set of numbers. doctors often use a certain number as a cutoff for a positive result. for example, your doctor may say the cutoff is 1 out of 200. this means that if your result is 1 out of 200 or 1 out of a number less than 200 (such as 1 out of 100), you have a positive result and your baby has a higher chance of a birth defect. if your result is 1 out of 300, this means that you have a negative result and your baby has a lower chance of a birth defect.

with the quad test, there is a chance of getting a false-positive test result. this means that the test could show a problem when the baby doesn't have the problem.

a false-positive result can cause stress and lead to tests you don't need (such as an amniocentesis). many women who have a positive screening test result are actually carrying a healthy baby.

sometimes negative test results can be wrong too. they may show that the baby is fine when he or she does have a birth defect. (this is a false-negative test result.)

your doctor will use your age and your baby's age to interpret the test results.