The ability to screen for Down syndrome is now available to all expectant parents and can be preformed as early as nine weeks of pregnancy. While it is a completely optional screening, it is one that can be an important part of routine prenatal care. With your doctor, you can discuss the advantages and disadvantages of screening, the types of tests available, the benefits, and the risks.
First Trimester Screening
Non-invasive prenatal testing consists in a series of screening tests. A screening test does not determine whether or not your baby will be born with Down syndrome – it only identifies the risk for the condition. The results of this test can help you determine whether you choose to have more specific genetic testing performed.
During your first trimester, the first screening test is a common blood test that measures the level of pregnancy-associated plasma protein-A (PAPP-A) and the pregnancy hormone known as human chorionic gonadotropin (HCG). If these levels are abnormal, this may indicate that further testing is needed.
The second screening in the first trimester is a type of ultrasound testing called the nuchal translucency (NT) test. This test uses an ultrasound to measure an area on the back of the fetus’ neck. If it appears that there’s more fluid than usual in this area, an abnormality may be present. During the ultrasound, your doctor can also check the fetus for “soft markers.” Soft markers do not provide a definitive diagnosis of Down syndrome, but may indicate a chromosomal abnormality. Soft markers can include things like: a shortened femur length, choroid plexus cysts, intracardiac echogenic foci (mineralization in the heart muscle), echogenic bowel, or a single umbilical artery.
Second Trimester Screening
If further testing is required after the first trimester, another blood test called quad screening may be performed during the second trimester. This tests for four pregnancy-associated markers: alpha fetoprotein, estriol, HCG, and inhibin A. Unusual levels of these substances may suggest a chromosomal abnormality. Using these tests, your doctor will determine the risk that your baby may have Down syndrome.
If the screening suggests a likelihood of Down syndrome, a diagnostic test can be performed. Prenatal diagnostic testing for Down syndrome requires a sample of genetic material. Once obtained, the sample is checked for the amount of material from chromosome 21, which indicates whether Down syndrome is present. Results can take 1-2 weeks to come back to your obstetrician. To get this sample of genetic material, one of the following tests can be performed:
Amniocentesis: Your doctor takes a sample of amniotic fluid that surrounds the fetus in the uterus during the 14th to 18th week of pregnancy.
Chorionic villus sampling (CVS): With CVS, your doctor takes a sample of cells from the placenta. This is done during the 9th to 11th week of pregnancy.
These procedures do carry a minor risk to the mother and fetus, including a miscarriage risk of 1-2%. If you are considering prenatal diagnostic testing, discuss the risks and benefits with your doctor beforehand.
If you receive a positive result on a Down syndrome screening or diagnostic test, your doctor will help you to thoroughly understand results, and will be able to provide a referral for an appointment with a genetic counselor to better understand what the results mean.