What is the quad screen?
The quad screen (sometimes referred to as the quadruple screen, AFP4, or multiple marker) is a blood test that can tell you whether your baby is at an increased risk for certain problems. The conditions it screens for are Down syndrome and trisomy 18, which are chromosomal abnormalities, as well as neural tube defects, such as spina bifida.
The quad screen is done in the second trimester, usually between 15 and 20 weeks of pregnancy. Ideally, the test should be performed in conjunction with first-trimester screening tests. This combination is known as integrated or sequential screening. But if you miss the opportunity to start your screening in the first trimester, the quad screen alone can still give you some good information.
These screening tests help you assess your baby's chances of having certain genetic diseases and birth defects. The results can help you decide whether to have amniocentesis, an invasive diagnostic test, to find out for sure about your baby's condition.
Screening tests tell you how likely it is that your baby has a condition, but only a diagnostic test like amniocentesis tells you whether your baby actually has the condition. While the quad screen carries no risk for the mom or baby, amniocentesis does have a small risk of miscarriage.
How can I decide which tests are right for me?
Obstetricians and Gynecologists now recommends that women of all ages be offered first- and second-trimester screening and diagnostic tests. Your practitioner should discuss the pros and cons of the various options with you.
If you need further information, consider meeting with a genetic counselor or a maternal-fetal medicine physician. But ultimately, whether to test – and which tests to have – is a personal decision.
Many women opt for screening first and then make a decision about diagnostic testing based on the initial results. Other women opt for diagnostic testing right away. (They may know that their baby is at high risk for a chromosomal problem or a problem that can't be detected by screening. Or they may just want to know as much as possible about their baby's condition and are willing to live with the small risk of miscarriage to find out.) Some women end up deciding to have no screening or diagnostic testing.
How is the screening done?
The quad screen is usually done between 15 and 20 weeks of pregnancy. You'll have a blood sample taken and sent to a lab for analysis. Results are usually available in about a week.
The test measures the levels of four substances in your blood:
High AFP can mean several things. A baby produces AFP throughout gestation, and a certain amount of it should cross the placenta into the mother's bloodstream at each stage.
If there's more AFP than expected, it may mean that you're carrying more than one baby or that your baby is older than your practitioner thought. But in some cases, it's a sign of an abnormal opening in the baby's spine (spina bifida), head, or abdominal wall that's allowing AFP to leak out. In rare cases, it can also signify a problem with the baby's kidneys. And in some cases, it doesn't mean any of those things.
Low AFP, low estriol, high hCG, and high inhibin A are associated with a higher risk for Down syndrome. Low levels of the first three means your baby has a higher than normal risk of having trisomy 18, a more severe and less common chromosomal abnormality.
Certain results may also indicate that you are at a somewhat greater risk for problems such as preeclampsia, premature birth, intrauterine growth restriction, or miscarriage. Knowing this can enable you and your practitioner to be on the lookout for signs of trouble.
How are the results computed?
To determine your baby's chances of having Down syndrome or trisomy 18, your levels of the four substances are plugged into a formula along with your baby's gestational age and your age.
Your age is factored in because although anyone can have a baby with a chromosomal abnormality, the risk increases as you age. For example, your likelihood of carrying a baby with Down syndrome ranges from about 1 in 1,200 at age 25 to about 1 in 100 at age 40.
To determine your baby's chances of having a neural tube defect, your level of AFP is plugged into a formula along with your age, your baby's gestational age, and other factors.
If you're having the quad screen as part of an integrated or sequential screening, the results will be combined with those from the tests you took in your first trimester for a more comprehensive risk assessment and a higher detection rate.