The quad screen — also known as the quadruple marker test, the second trimester screen or simply the quad test — is a prenatal test that measures levels of four substances in pregnant women's blood:

• Alpha-fetoprotein (AFP), a protein made by the developing baby
• Human chorionic gonadotropin (HCG), a hormone made by the placenta
• Estriol, a hormone made by the placenta and the baby's liver
• Inhibin A, another hormone made by the placenta

Ideally, the quad screen is done between weeks 15 and 18 of pregnancy — during the second trimester. However, the procedure can be done up to week 22.

The quad screen is used to evaluate whether your pregnancy has an increased chance of being affected with certain conditions, such as Down syndrome or neural tube defects. If your risk is low, the quad screen can offer reassurance that there is a decreased chance for Down syndrome, trisomy 18, neural tube defects and abdominal wall defects.

If the quad screen indicates an increased chance of one of these conditions, you might consider additional screening or testing.

The quad screen evaluates your chance of carrying a baby who has any of the following conditions:

• Down syndrome (trisomy 21). Down syndrome is a chromosomal disorder that causes lifelong intellectual disability and developmental delays and, in some people, health problems.
• Trisomy 18. This is a chromosomal disorder that causes severe developmental delays and abnormalities in the structure of the body. Trisomy 18 is often fatal by age 1.
• Spina bifida. Spina bifida is a birth defect that occurs when a portion of the neural tube fails to develop or close properly, causing defects in the spinal cord and in the bones of the spine.
• Abdominal wall defects. In these birth defects, the baby's intestines or other abdominal organs stick through the belly button.

The quad screen has traditionally been one of the most commonly used screenings in the second trimester. It was generally used if prenatal care began during the second trimester or if first trimester screening, which involves a blood test and an ultrasound exam, wasn't available. Your health care provider might combine the results of first trimester screening with the quad screen to improve the detection rate of Down syndrome.

Prenatal cell-free DNA screening is another screening method that your health care provider might recommend in place of quad screening. Talk to your health care provider about your screening options.

A negative quad screen doesn't guarantee that the baby won't have a chromosomal abnormality, single-gene disorder or certain birth defects. If your screening test is positive, your doctor will recommend additional testing to make a diagnosis.

Before the screening, think about what the results mean to you. Consider whether the screening will be worth any anxiety it might cause, or whether you'll handle your pregnancy differently depending on the results. You might also consider what level of risk would be enough for you to choose a more invasive follow-up test.

The quad screen is a routine prenatal screening test. The test poses no risk of miscarriage or other pregnancy complications.

As with other prenatal screening tests, however, the quad screen can cause anxiety about the possible test results and what they might mean for your baby.

Before the test, your health care provider might ask you to meet with a genetic counselor. Or your health care provider might provide genetic counseling during your routine prenatal care appointment.

You can eat and drink normally before the test.

During the quad screen, a member of your health care team takes a sample of blood by inserting a needle into a vein in your arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.

The quad screen measures levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG), estriol and inhibin A in pregnant women'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 give the level of risk of carrying a baby who has certain conditions compared with the general population's risk. 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 how long you've been pregnant
• Maternal race
• Maternal weight
• Carrying more than one baby during a pregnancy
• Diabetes
• 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 you're carrying.

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, neural tube defects or abdominal wall 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 might cause you to consider other testing, such as:

• Prenatal cell-free DNA screening. This sophisticated blood test examines cell-free DNA from the placenta and the fetus in the mother's bloodstream. It 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 this test. 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, 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 and neural tube defects. During amniocentesis, 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 might mean for your pregnancy.