Combined First Trimester Ultrasound and Serum Screening for Birth Defects
What Is the Combined First Trimester Ultrasound and Serum Screening Test for Birth Defects?
A combination of an ultrasound examination and a blood test is available to all pregnant women in Connecticut. These tests are carried out between 10 1/2 weeks and 14 weeks of pregnancy (counting from the first day of the last menstrual period). The purpose of this test is to identify pregnancies with an increased chance for a birth defect, especially Down syndrome.
The ultrasound examination is performed only by specially trained individuals who measure the amount of fluid accumulation behind the neck of the developing baby (“nuchal translucency” measurement). The blood tests are carried out in the Genetics laboratory at UConn Health where they measure the concentrations of two specific substances: human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A (PAPP-A). In order to interpret the results, information is needed about the pregnant woman, including her age, weight, race and duration of the pregnancy at the time of testing.
The combined screening test provides information early in pregnancy. If the test result is normal, women and their partners can be reassured that the chance for Down syndrome is small. This reassurance can help relieve the anxiety experienced by many pregnant women.
How Is This Information Used?
The results of the ultrasound examination and the blood test are used to calculate a risk that a birth defect may be present. It is important to recognize that this evaluation is a “screening test”, i.e., it indicates whether the risk is high or low but does not provide a definite diagnosis.
A pregnant woman who has a normal ultrasound examination and blood levels of hCG and PAPP-A that fall within the expected ranges will be told that her chance to have a baby with Down syndrome is not high enough to consider any immediate follow-up testing. Such patients are considered to be “screen-negative.” This is not a guarantee that the unborn baby will be perfectly healthy. It does mean that there is no indication that the pregnancy is at high risk for Down syndrome. Additional screening tests will be offered later in pregnancy (15 to 18 weeks gestational age) to further evaluate the risk for birth defects.
When the testing indicates a “screen-positive” result, the patient will be informed that her pregnancy has a higher chance for Down syndrome or another disorder. Having a screen-positive result does NOT mean that the baby will have a problem. In fact, most patients with screen-positive results will have healthy babies. The screen-positive result indicates that further consideration should be given to follow-up testing that can help determine whether an abnormality is indeed present.
What Should I Do If My Test Is "Screen-Positive"?
Some women who receive a high first trimester risk may wish to consider chorionic villus sampling (CVS) to accurately diagnose Down syndrome. CVS involves testing on a tiny sample of the tissue (villi) that develops into the placenta. This test is usually carried out at 10 to 13 weeks gestational age. CVS is nearly 100% effective in making the diagnosis of Down syndrome. However, the procedure does carry a risk (about 1%) for causing a miscarriage.
Other women who receive a “screen-positive” result may prefer to have more tests during the second trimester. This can include additional blood screening tests (see Why do I need additional screening tests in the second trimester?).
Second trimester ultrasound evaluation may also be recommended. A detailed (high resolution) ultrasound examination allows the doctor to “look” at the baby’s heart, stomach, kidneys, bladder, arms, legs, fluid around the baby, and the placenta. This ultrasound exam is effective in identifying some of the physical features seen in some babies with Down syndrome.
To accurately diagnose Down syndrome in the second trimester, an amniocentesis can be considered. This involves sampling some of the fluid around the baby at 16 to 18 weeks gestation. Amniocentesis is also nearly 100% effective in making the diagnosis of Down syndrome and has a small (less than 0.5%) risk for causing a miscarriage.
Four tests, alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), unconjugated estriol (uE3) and Inhibin-A, that are performed on a blood sample obtained at 15 to 18 weeks gestation can also help to screen for Down syndrome.
If these tests are performed for a woman who has also received first trimester screening, it is essential that all tests, both the first trimester and second trimester components, are considered together to establish risk. When this is done, this combination of all tests provides the most accurate information and the lowest number of positive test results.
Many women who are “screen-positive” by first trimester screening will be found to be at low risk following the completion of all tests. However, there will be some women who are initially found to be at low risk but are later found to be “screen-positive” after all tests are complete. It is important to remember that the most accurate risk figure will be the one that includes all tests performed.
One of the second trimester tests, AFP, is also used to screen for additional birth defects called open neural tube defects (anencephaly and spina bifida). A small proportion of women who are found to have a low risk for Down syndrome will have an increased risk for these other disorders. A detailed second trimester ultrasound exam is often helpful for these women.
What Is Down Syndrome?
Down syndrome is a condition that results from the presence of an extra chromosome in the fertilized egg. Chromosomes are the packages of genes, the genetic blueprints for development.
Typically, in each human cell, there are 46 chromosomes. Individuals with Down syndrome have 47 chromosomes per cell; they have an extra chromosome number 21. This results in a specific pattern of features including characteristic appearance, mental retardation, delayed development and increased risk for certain birth defects, such as congenital heart disease.
Any woman may have a baby with Down syndrome at any age. However, the chance to have a baby with Down syndrome gets progressively higher as a woman gets older. For example, at age 35, the chance to have a baby with Down syndrome is approximately 1:270 in the second trimester. When the blood test is carried out, the mother’s age is combined with the ultrasound and laboratory data to calculate her risk. Risks greater than or equal to 1:270 are considered to be “screen-positive” for Down syndrome.
What Else Do I Need to Know About This Test?
The Combined First Trimester Ultrasound and Serum Screening for Birth Defects will identify a number of other birth defects in addition to Down syndrome. These include cardiac defects, other chromosome abnormalities such as trisomy 18, and a variety of other rarer genetic disorders.
Approximately one out of every fifteen women screened will have a "screen-positive" blood test result; the vast majority of these women will go on to have healthy babies. Despite this reassuring data, many women understandably become quite anxious when they are informed that their blood test results are “abnormal.” Your doctor will provide you with more detailed information about your individual results.
It is also important to remember that because this is a “screening” test, it is possible to have a normal result but still have a baby with Down syndrome or other genetic condition. Approximately 85 to 90% of Down syndrome pregnancies are identified through this testing.
Genetic counselors are trained professionals who can also provide detailed information about the combined ultrasound and serum screening test for birth defects to patients and their health care providers. The genetic counseling staff at the University of Connecticut Health Center welcomes your questions.
We are available for consultation both by telephone 860-679-3387 and appointment. Our office is open weekdays from 8 a.m. to 4:30 p.m.
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