Quad screen

Definition

The quad screen is a prenatal test that measures levels of four substances in a pregnant woman's blood:

• Alpha-fetoprotein (AFP), a protein made by the baby's liver
• 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

Typically, the quad screen is done between weeks 15 and 20 of pregnancy. Results of the quad screen indicate your risk of carrying a baby who has certain developmental or chromosomal conditions, such as spina bifida or Down syndrome — not whether your baby actually has these conditions.

If your risk level is low, the quad screen can offer reassurance of a healthy pregnancy. If your risk level is moderate or high, you may choose to follow the quad screen with more invasive diagnostic testing — such as amniocentesis — to determine whether the baby actually has Down syndrome or other developmental or chromosomal conditions.

The quad screen is also known by various other names, including the quad marker screen and four marker screen.

Why it's done

The quad screen is done to evaluate your risk of carrying a baby who has certain developmental or chromosomal conditions, including:

• Spina bifida. Spina bifida is a serious birth defect that occurs when the tissue surrounding a baby's developing spinal cord doesn't close properly.
• Anencephaly. Anencephaly is an underdeveloped brain and an incomplete skull. A baby born with anencephaly may be stillborn or die within a few hours or days after birth.
• Down syndrome (trisomy 21). Down syndrome is a genetic condition that causes mental retardation and other medical problems.
• Edwards syndrome (trisomy 18). Edwards syndrome is a condition that causes severe mental retardation. Edwards syndrome is often fatal by age 1.

A similar test known as the triple screen — which only checks AFP, HCG and estriol — is sometimes offered in place of the quad screen. The quad screen is more sensitive, however. Some health care providers choose to combine the results of first trimester screening — a two-step test that includes a blood test and a specialized ultrasound exam — with the quad screen. Although first trimester screening is done earlier in pregnancy than is the quad screen, you won't learn the final results of this sequential testing until both tests are completed and analyzed.

Remember, the quad screen is optional — and test results only indicate whether you have an increased risk of carrying a baby who has certain developmental or chromosomal conditions, not whether your baby actually has the condition. Before the screening, think about what the results mean to you. Consider whether the screening will be worth any anxiety it may 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. If cost is a concern, ask your insurance provider for coverage details.

Risks

The quad screen is a routine prenatal screening test. The test poses no risk of miscarriage or other pregnancy complications. However, it can be stressful to wait for test results or anticipate the need for invasive follow-up testing.

How you prepare

You don't need to do anything special to prepare for the quad screen. You can eat and drink normally before the blood test.

What you can expect

During the quad screen, a member of your health care team simply 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. Test results are typically available within a week.

Results

Your health care provider will use your age and the results of the quad screen to gauge your risk of carrying a baby who has certain developmental or chromosomal conditions. Other factors, such as your personal or family health history, also may affect your risk. Your health care provider or a genetic counselor will help you understand this risk and what it means for your pregnancy.

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
• Maternal smoking

If your test results are positive, your health care provider may 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 developmental or chromosomal conditions. A low-risk result doesn't guarantee that your baby won't have one of these conditions. Likewise, a high-risk result doesn't necessarily mean that your baby will be born with one of these conditions.

Often, abnormal test results are an invitation to consider further diagnostic testing, such as amniocentesis. This test is more invasive than is the quad screen and carries a slight risk of miscarriage, but it gives more specific information about your baby's health. Although ultrasound can often be used to detect spina bifida and anencephaly, an invasive test is the only way to diagnose chromosomal disorders.

The bottom line: Discuss the test results with your health care provider. A genetic counselor also can help you understand your options.