Almost all babies are born healthy in the United States, however 2 or 3 out of every 100 babies are born with a major birth defect. There are several screening options, noninvasive and invasive, available during pregnancy.
First Trimester Screen: Done between 11 weeks 6 days and 13 weeks 6 days. This test is a blood test (measuring PAPPA and HCG) as well as an ultrasound that measures the nape of your baby’s neck. The results are combined and an estimated risk is then given for Down syndrome, trisomy 18 and trisomy 13.
If you are found to have a positive screen or increased risk of one of these conditions, your physician will then discuss additional testing that is available to you. These can include more invasive diagnostic tests, which will be discussed later.
Free Cell DNA testing: This is a blood test (done any time after 10 weeks) that measures the relative amount of chromosome material in the maternal blood. Some of the conditions that the test is looking for include: Down syndrome, Trisomy 18, Trisomy 13. This test is approximately 99% accurate in detecting these conditions.
Alfa-fetoprotein: This is a blood test done between 15 and 20 weeks 6 days, if elevated, this indicates an increased risk for neural tube defects like spina bifida. It is 80-85% accurate.
Detailed Ultrasound: This is a detailed ultrasound that can be done for women with certain risk factors. It is done to screen for abnormalities that may indicate that the fetus has a birth defect.
Diagnostic tests evaluate the sample of cells from the developing fetus to determine the actual number of chromosomes. These tests can detect Down syndrome and Trisomy 18 in 100% of cases, but are associated with an increased risk of miscarriage and infection. All of the outlined tests are available for all women but are optional. Insurance coverage varies for everyone and depends on your particular health plan. Please ask your physician if you have any additional questions. Both of these procedures are done by a perinatologist.
1st trimester (11-13.6 weeks): Chorionic villus sampling (CVS). During this procedure, a needle is passed through the abdomen or the vagina to withdraw a small sample of cells from the placenta. This procedure is performed with ultrasound guidance. The cells are then analyzed to determine the actual number of chromosomes and can detect if there are extra chromosomes present. The chance of miscarriage after this procedure is approximately 0.5-1%.
2nd trimester (14-20 weeks): Amniocentesis. During this procedure, a needle is passed through the abdomen and into the uterus and a small amount of amniotic fluid is withdrawn. This procedure is performed with ultrasound guidance. Cells from the fetus that are found in the amniotic fluid are analyzed to determine the actual number of chromosomes and can detect if there are extra chromosomes present. The chance of miscarriage after this procedure is approximately 0.5%.
1. PerkinElmer Labs (2013) First Trimester Screen for chromosome abnormalities including Down syndrome, N.P.
2. ACOG (American Congress of Obstetricians and Gynecologist) (2015) Screening Tests for Birth Defects. Retrieved from http://www.acog.org/Patients/FAQS/Screening-Tests-for-Birth-Defects