Genetic Screening Tests for Women 35 or Older
Quad Marker Screen
A quad marker screen is a blood test performed between the 15th and 20th weeks of pregnancy. It measures substances in the blood that may show:
- Problems with a baby's brain and spinal cord, called neural tube defects. These include spina bifida and anencephaly. The quad marker screen can detect about 75% to 80% of neural tube defects.
- Genetic disorders such as Down syndrome. The test can detect about 75% of Down syndrome cases in women under age 35 and more than 80% of Down syndrome cases in women 35 and older.
It's important to know that the quad screen only indicates your level of risk for birth defects. If the test shows a risk greater than the average risk of a 35-year-old woman, then the test is considered to be positive. The test can't diagnose birth defects, so a positive result doesn't mean that your babies have a birth defect. In most cases, the babies are healthy despite an abnormal test result.
If the test is positive, your doctor may suggest having diagnostic tests, such as:
- Amniocentesis to check your babies' chromosomes
- Ultrasound to look for signs of birth defects
Unlike blood testing, which only shows whether you are at risk, amniocentesis is used to make a diagnosis. During the test, your doctor will insert a needle through the abdomen wall, using ultrasound images to help guide the needle into the uterus. He or she will remove a small amount of amniotic fluid from the sac surrounding your babies. This sample is then used to check the babies' chromosomes and test for genetic diseases. In addition to the most common chromosome problems, including Down syndrome, trisomy 18, trisomy 13, and Turner syndrome, the sample can also be tested for:
- Sickle cell disease
- Cystic fibrosis
- Muscular dystrophy
- Tay-Sachs disease
- Neural tube defects, such as spina bifida and anencephaly
Chorionic Villus Sampling (CVS)
CVS is an alternative to amniocentesis, and it can be performed earlier in the pregnancy. Like amniocentesis, CVS can diagnose some diseases. If you have certain risk factors, you may be offered CVS as a way to detect birth defects during early pregnancy.
During this test, a small sample of cells, called chorionic villi, is taken from the placenta. Chorionic villi are tiny parts of the placenta that are formed from the fertilized egg, so they have the same genes as your babies. To get the cells, a doctor passes a needle through the vagina or the abdomen, depending on the location of the placenta. The cells are used to check your babies' chromosomes, just like amniocentesis.
CVS and amniocentesis usually involve genetic counseling, where you speak with a counselor about your risk for genetic disorders. You will also learn about the risks and benefits of the procedures.