Genetic Screening in Southern New England
The quad marker screen is a blood test that provides a woman and her health care provider with useful information about her pregnancy. The quad marker screen might be performed between 15 and 20 weeks of pregnancy.
During the quad marker screen, a sample of blood is taken from the woman’s vein. Substances in the blood sample are measured to screen for:
- Problems in the development of the fetus’ brain, spinal cord, and other tissues of the central nervous system (neural tube). Problems with neural tube development can occur as spina bifida or anencephaly (absence of all or part of the brain). Neural tube defects occur in one or two out of every 1,000 births. The quad marker screen can detect approximately 75 percent of open neural tube defects.
- Genetic disorders such as Down syndrome, a chromosomal abnormality — Approximately one in 720 babies is born with Down syndrome. The quad marker screen can detect approximately 75 percent of Down syndrome cases in women under 35 and 85 to 90 percent of Down syndrome cases in women 35 and older.
A quad marker screen might be offered to you by your health care provider between 15 and 20 weeks of pregnancy, counting from the first day of your last menstrual period.
The blood sample is sent to a laboratory and tested for the presence of the following four substances, which are normally found in the baby’s blood, brain, spinal fluid and amniotic fluid:
- Alpha-fetoprotein (AFP) — A protein produced by the fetus’ liver
- Unconjugated estriol (UE) — A protein produced in the placenta and the fetus’ liver
- Human chorionic gonadotrophin (hCG) — A hormone produced by the placenta
- Inhibin-A — A hormone produced by the placenta
The expected amount of these substances normally found in the mother’s bloodstream changes each week of pregnancy, so it is important to tell your health care provider how far along you are in your pregnancy. Higher-than-normal AFP levels could indicate that the fetus has an open neural tube defect. High AFP levels might also indicate that the fetus is older than was thought or that the woman is expecting twins. Lower-than-normal AFP levels could indicate that a woman is at higher risk for having a baby with Down syndrome.
Levels of hCG and Inhibin-A are higher than normal when a woman has an increased risk of having a baby with Down syndrome. Lower than normal levels of estriol might also indicate that a woman is at high risk for having a baby with Down syndrome.
Yes. The quad marker screen is a safe and useful screening test for families concerned about birth defects or genetic disorders. It is a test that carries no risk to the baby, since a blood sample is taken only from the mother.
Normal levels of AFP, UE, hCG and Inhibin-A strongly indicate that you have a healthy pregnancy and a healthy baby. In more than 98 percent of pregnancies, normal quad marker screen results predict healthy babies and births without major complications. However, there are no prenatal tests that can guarantee your baby and pregnancy will be completely healthy or without complications.
Quad marker screen results that are not in the normal range do not necessarily mean that there is a problem with your pregnancy.
The quad marker screen is used for screening only, which means it can only assess your risk of having a baby with a certain birth defect. It is not used to diagnose the particular problem that might be present. If the quad marker screen results are not in the normal range, further tests, such as ultrasound or amniocentesis might be necessary.
Out of 1,000 pregnant women, approximately 50 will have quad marker screen results that indicate an increased risk for having a baby with an open neural tube defect. Of those 50 women, only one or two will actually have a baby with an open neural tube defect. About 40 women will have quad marker screen results that show an increased risk for having a baby with Down syndrome, and one or two will actually have a baby with Down syndrome.
We recommend that all pregnant women have a quad marker screen, but it is your decision whether or not to have the test. However, if you have any of the following risk factors, you might want to consider having the test:
- You are age 35 or older
- Your family has a history of birth defects
- You’ve had a previous child with a birth defect
- You have had insulin-dependent (type 1) diabetes before your pregnancy