Prenatal testing plays a crucial role in assessing the health of your baby during pregnancy. For those expecting, understanding the options available for Down syndrome testing and diagnosis is essential. This guide provides a detailed overview of both screening and diagnostic tests, helping you make informed decisions throughout your pregnancy journey.
Prenatal Screening Tests for Down Syndrome
Screening tests are non-invasive methods designed to estimate the risk of a baby having Down syndrome (Trisomy 21) and other chromosomal conditions. In California, the California Prenatal Screening (CA PNS), also known as sequential integrated screening, is offered to all pregnant individuals. This process involves two steps:
First Trimester Screening (10-14 weeks):
This initial step combines a maternal blood test with a nuchal translucency (NT) ultrasound.
- Maternal Blood Test: A blood sample is taken from the pregnant person to measure specific substances in the blood.
- Nuchal Translucency (NT) Ultrasound: This specialized ultrasound measures the clear (translucent) space at the back of the baby’s neck. Fetuses with Down syndrome and certain other conditions tend to have increased fluid accumulation in this area, leading to a larger NT measurement.
The results from the blood test, the NT measurement, and the pregnant person’s age are combined to calculate a risk assessment for Down syndrome and trisomy 18 (Edwards syndrome). It’s important to remember that screening tests do not provide a definitive diagnosis; they only indicate the likelihood of these conditions.
Second Trimester Screening (15-20 weeks):
The second step involves another maternal blood test. When the results of this blood test are integrated with the first trimester screening results (blood test and NT ultrasound), the detection rate for Down syndrome significantly improves. This combined screening also offers a risk assessment for:
- Trisomy 18
- Smith-Lemli-Opitz syndrome (SLOS), a genetic disorder affecting development.
- Open neural tube defects (ONTD), such as spina bifida, which are birth defects of the brain, spine, or spinal cord.
- Abdominal wall defects, where there is an opening in the baby’s abdomen.
It is vital to understand that a positive screening result indicates an elevated risk, not a confirmed diagnosis. If a screening test suggests a higher chance of Down syndrome, diagnostic tests are available to provide a definitive answer.
Diagnostic Tests for Down Syndrome
Diagnostic tests are more invasive procedures that can provide a definitive diagnosis of Down syndrome and other chromosomal abnormalities. The primary diagnostic tests are amniocentesis, chorionic villus sampling (CVS), and, to a lesser extent, ultrasound.
Amniocentesis:
Amniocentesis is a commonly used diagnostic test to identify chromosomal problems, including Down syndrome. It is typically performed between 15 and 20 weeks of pregnancy.
- Procedure: Under ultrasound guidance, a thin needle is inserted through the abdomen into the amniotic sac to withdraw a small sample of amniotic fluid. This fluid contains fetal cells.
- Analysis: The cells from the amniotic fluid are cultured in a lab, and a karyotype test is performed. A karyotype is an analysis of the cells’ chromosomes, allowing for the detection of extra or missing chromosomes, like the extra chromosome 21 that causes Down syndrome.
- Results: Results are usually available in about two weeks. Amniocentesis is highly accurate in detecting Down syndrome and most other chromosomal disorders.
- Risks: There is a small risk of miscarriage associated with amniocentesis, estimated to be around 1 in 900. At specialized centers like UCSF, the miscarriage rates associated with amniocentesis are reported to be exceptionally low.
Chorionic Villus Sampling (CVS):
Chorionic Villus Sampling (CVS) is another diagnostic test used to identify chromosomal abnormalities like Down syndrome, and it can be performed earlier in pregnancy than amniocentesis, typically between 10 and 13 weeks.
- Procedure: CVS involves taking a small tissue sample from the placenta (chorionic villi). Under ultrasound guidance, the sample can be obtained through the abdomen using a needle or through the vagina and cervix using a catheter.
- Analysis: Similar to amniocentesis, the tissue sample is cultured, and a karyotype test is performed to analyze the chromosomal makeup of the fetal cells.
- Results: Results are also typically available within two weeks.
- Advantages & Disadvantages: CVS offers the advantage of earlier diagnosis compared to amniocentesis. However, CVS does not detect neural tube defects like spina bifida. Screening for neural tube defects can be done later in pregnancy through blood tests (expanded alpha-fetoprotein or AFP) or ultrasound.
- Risks: CVS carries a slightly higher miscarriage risk than amniocentesis, estimated to be about 1 in 450. Similar to amniocentesis, centers with expertise, like UCSF, report very low miscarriage rates associated with CVS procedures.
Ultrasound:
While primarily used to assess pregnancy status (gestational age, fetal size, number of fetuses), ultrasound can also provide some indicators of potential birth defects, including some associated with Down syndrome.
- Role in Diagnosis: A detailed ultrasound exam is typically performed before any invasive diagnostic tests are considered. Specific ultrasound findings may raise suspicion for Down syndrome, prompting further diagnostic testing.
- Limitations: Ultrasound alone cannot definitively diagnose Down syndrome. It is a valuable tool for screening and for guiding invasive procedures, but it is not a standalone diagnostic test for chromosomal abnormalities.
Understanding a Positive Screening or Diagnostic Result
Positive Screening Result:
If your screening test results indicate a higher risk for Down syndrome, it is crucial to discuss these results and your options with your healthcare provider and a genetic counselor. They can explain the implications of the results in detail and discuss the availability of diagnostic testing. The decision to proceed with diagnostic testing is entirely personal.
Positive Diagnostic Result:
If a diagnostic test confirms a genetic abnormality such as Down syndrome, it is recommended to have in-depth discussions with specialists. This includes medical geneticists and genetic counselors, in addition to your doctor. These experts can provide comprehensive information about Down syndrome, potential health implications, and available resources and support.
Making informed decisions about prenatal testing and understanding the results is a significant part of prenatal care. Consulting with your healthcare provider and genetic counselors will empower you to navigate this process with knowledge and confidence.
