Who Performs This Test?
To perform first-trimester screening the physician must undergo specialized training that is monitored by international and national accreditation organizations. Currently, there are three levels of testing. Dr. DeVore is one of the few physicians in southern California certified at all three levels.
Level I. Basic Examination
To determine the risk for Down syndrome an ultrasound examination is performed in which the length of the fetus (crown-rump length) is measured to determine its age. Following this, the nuchal translucency is measured which consists of the thickness of the fluid space behind the neck. After these measurements are obtained, blood is drawn from the patient to measure two proteins Pregnancy-Associated Plasma Protein A (PAPP-A) and human chorionic gonadotropin. The measurements from the ultrasound examination, coupled with the results of the blood test, are combined to calculate a risk for Down syndrome and Trisomy 18.
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Level II. Basic Examination plus Nasal Bone
Additional ultrasound training and accreditation are required by an international organization to identify the presence or absence of the nasal bone. The absence of the nasal bone is a strong predictor for an increased risk for Down syndrome and other chromosomal defects. This examination is more difficult than the Level I study because it requires the physician to identify two small nasal bones. Fetuses with Down syndrome may demonstrate either absence of the nasal bones, or underdevelopment of one or both bones. During the first-trimester a normal study would consist of the identification of the nasal bones, while an abnormal study would be the absence of the nasal bones. If the nasal bones are absent, this significantly increases the risk for Down syndrome. The identification of nasal bones is also useful when a patient is informed that her risk for Down syndrome is increased following a Level 1 examination. If the nasal bone is identified in a Level II evaluation, then the risk reported following the Level 1 examination can be further decreased.
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Level III. Basic Examination plus Nasal Bone plus Evaluation of the Heart
Additional training and accreditation are required by an international organization to identify the presence or absence of tricuspid regurgitation. This occurs when blood flows back into the right atrium of the fetal heart when the ventricles contract. If this is present, there is an increased risk for Down syndrome and other chromosomal abnormalities. In addition, blood flow through the ductus venosus, a vessel that returns blood back to the heart from the placenta, can also be evaluated. If abnormal blood flow is identified, this increases the risk for Down syndrome and other chromosomal abnormalities. Several years ago Dr. DeVore was the first to report the association between tricuspid regurgitation and Down syndrome when present during the second-trimester of pregnancy. Subsequent studies in first-trimester fetuses have also confirmed this association. Therefore, if tricuspid regurgitation is present, the risk for Down syndrome increases.
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What are the Possible Interpretations of the Result from the First-Trimester Screening Test ?
Scenario 1. Your risk for Down syndrome is higher than 1 in 30, i.e. 1 in 20, 1 in 10, etc. Because the risk for Down syndrome is quite high, you would be offered chorionic villus sampling (CVS).
Scenario 2. Your risk is between 1 in 30 and 1 in 210. This is still considered high-risk for a chromosomal abnormality. You would be offered either a 1st trimester CVS test or a 2nd trimester amniocentesis; the latter having a lower risk than CVS. However, one consideration is the risk you started with before the test was performed. For example, if your initial risk was 1 in 30 because of your age, and decreased to 1 in 200, this is a marked improvement. However, the risk of 1 in 200 may still be interpreted as "high-risk" because it is the cutoff for offering amniocentesis.
Scenario 3. Your risk is lower than 1 in 210. For example, 1 in 300, 1 in 500, 1 in 1,000, etc.. You would not be offered invasive testing but would be informed that the first-trimester test has eliminated a certain percentage of fetuses with Down syndrome and Trisomy 18. For those fetuses that were not detected, Dr. DeVore would invite you to return at 17 to 21 weeks for Genetic Ultrasound. This is a specialized ultrasound examination, when performed by Dr. DeVore, which identifies 97% of the remaining fetuses with Down syndrome, 98% of fetuses with Trisomy 18 and Trisomy 13, as well as other serious birth defects.
What Should You Do If You Desire to Undergo First-Trimester Screening?
Step 1. Call 626-583-8911 to make an appointment to be seen by Dr. DeVore. The evaluation should be done between 11 and 13 weeks 6 days from your last menstrual period, or 9 and 11 weeks 6 days from conception, i.e. IVF.
Step 2. Prior to the examination Dr. DeVore will review with you the purpose of this test. This will include a discussion of the detection rate of 1st trimester screening for Down syndrome and other birth defects.
Step 3. Initially, the examination will be performed by placing the ultrasound transducer on your abdomen. Depending upon the ability to visualize the fetus, a vaginal ultrasound examination may be required. Occasionally, certain components of the examination may not be obtained because of fetal position. If this occurs, Dr. DeVore will discuss the implications with you.
Step 4. Blood will be drawn to measure PAPP-A and beta-human chorionic gonadotropin. The results of the ultrasound will be sent to the laboratory with the blood sample. The risk for Down syndrome and Trisomy 18 will be reported to you within a few days.