Screening tests are designed to provide information about whether or not a pregnancy is at an increased risk to be affected with a chromosome condition. Screening tests never tell for sure if a pregnancy is, or is not, affected with Down syndrome or another chromosome condition.
Some screening tests also detect certain isolated birth defects, such as openings on the spine (spina bifida) or head (anencephaly), cleft lip and palate, heart defects, and/or problems with other organs.
These tests include cell free DNA (NIPS) screening, first trimester screening, maternal serum screening, and ultrasound.
Prenatal Screening Information:
1. Non-Invasive Prenatal Screening/Testing (cell free DNA, also known as NIPS and NIPT)
Cell free DNA (cf DNA) uses the maternal blood to screen for trisomy 21, 13, 18 and sex chromosome abnormalities and is offered by several different laboratories. This test analyzes the cell-free placental DNA that circulates in a woman's blood when she is pregnant. cfDNA is most useful in pregnancies that are at increased risk for a fetal chromosome abnormality (such as due to maternal age, abnormal screening tests/ultrasounds). There is controversy whether cfDNA should be used in the low or average risk population at this time. Maternal blood can be drawn for this test beginning at 10 weeks gestation, but is typically drawn at the time of the first trimester ultrasound, which is used to measure the nuchal translucency (NT), and is performed between 11-13 6/7 weeks. The results are available in ~10 days and are called to the patient by our office. It is important to note, that while cfDNA has the the highest detection for Downs when compared to other screening test available, cfDNA cannot replace the detection and accuracy of a complete chromosome analysis that can only be obtained through a CVS or amniocentesis.
2. First Trimester Screening and Integrated Screening
First trimester screening is typically performed for pregnancies that do not have any known risk factors to be affected with chromosome abnormalities (maternal age below 35, no abnormal ultrasound findings, etc) to better estimate the specific chance/probability for the pregnancy to be affected with Down syndrome, trisomy 13, and/or trisomy 18. The test cannot diagnose or definitively rule out these conditions, but instead determines if the pregnancy is at a normal or increased risk. If the test result shows an increased risk, the patient will be offered additional testing to better assess the risk/and or provide a definitive diagnosis. Integrated screening incorporates the first trimester screen with a second blood draw. Your insurance may affect which test is available to you.
3. Maternal Serum Screening
Maternal serum screening (i.e. multiple marker, AFP tetra) is performed when first trimester screening was not performed/unavailable. It is generally not recommended to have both tests, as the first trimester screen has a higher detection rate and lower false positive rate. For any patient who has not had the first trimester screening, the maternal serum screen can be used to identify pregnancies that are at an increased risk to be affected with either Down syndrome, trisomy 18, and open neural tube defects such as spina bifida or anencephaly.
4. Detailed/level II Ultrasound
Ultrasound in general is used to provide accurate dating of a pregnancy and to evaluate the fetus's physical growth and development. An ultrasound at 18-20 weeks of pregnancy, a detailed or level II ultrasound will measure and evaluate the bones and internal organs for signs of abnormalities. In addition to evaluating for birth defects, benign changes also known as soft markers are examined.
Some fetuses with Down syndrome, trisomy 18, trisomy 13, or other chromosome problems will have birth defects or other physical changes that can be seen on ultrasound. The level II ultrasound can detect about 60% of cases of Down syndrome, 99% of trisomies 13 and 18, and 97% of openings of the spine or head. A normal ultrasound does not guarantee that the pregnancy does not have one of the conditions; it only indicates that the pregnancy is at lower or higher risk.
5. Universal Genetic Carrier Screening
The Counsyl Family Prep Screen is a genetic test that parents have to see if they are a "carrier" for up to 176 different genetic diseases [such as cystic fibrosis (CF), spinal muscular atrophy (SMA), Fragile X, PKU, Tay-Sachs disease, etc]. ~1/400 babies are born with one of these diseases. According to medical guidelines, all couples should be offered carrier screening for CF and SMA. This new technology allows for additional disease to be screened for with the same test. Through this test, more pregnancies can be identified as high risk for a genetic condition and be offered additional testing/management options that would not have been routinely offered/available with the test.
Please see the Carrier Screening handout under our "Forms and Policies" page for additional details on this test. Visit the Counsyl website for more information.