415.202.1200
One Daniel Burnham Court, Suite 230C San Francisco, California 94109
Our Services
Screening Overview
Prenatal screening tests have been developed in an effort to establish the specific risk a fetus has for specific abnormalities, such as Down syndrome. The main benefit of screening tests is that they provide risk information without putting the pregnancy at risk for a complication such as a miscarriage. These tests use information from a pregnant woman’s blood and/or from a targeted ultrasound examination of the fetus, to determine whether the fetus is at risk for Down syndrome, other chromosome abnormalities, and/or specific birth defects. Screening tests can help to identify diseases such as cystic fibrosis (CF), hemoglobinopathies, and other genetic disorders that tend to run at a higher frequency in specific ethnic backgrounds. Screening tests can also provide risk information for specific structural birth defects, such as spina bifida.
It is important to remember that screening tests are not able to tell us with certainty if a disorder is actually present in a pregnancy; only what the risk is. For example, the result of a screening test will provide a risk number, such as 1 in 100, or 1%, chance that a pregnancy is affected with a specific abnormality, such as Down syndrome. This means that there is a 99 in 100, or 99%, chance that Down syndrome is not present in the pregnancy. This 1% chance is the adjusted individual risk assessment for the pregnancy. This information can then be used to help make decisions about diagnostic testing.
Diagnostic tests, such as chorionic villus sampling (CVS) and amniocentesis, are able to tell with certainty whether a chromosome abnormality is present in a pregnancy. These tests take a sampling of the fetal cells (cells from the baby) and evaluate the chromosome count and basic structure. Therefore, the results provide information about whether the chromosome count is 46, the typical number, or whether there are extra or missing chromosomes in the pregnancy. Diagnostic tests provide a “yes” or “no” answer rather than risk assessment for a pregnancy. The limitation of diagnostic testing is that there is a very slight risk for a complication such as a miscarriage.
It is important to understand that most of the time a pregnancy is not actually affected with a chromosome abnormality even when the individual risk assessment is increased over the background age-related risk. For example, a screening result may show that the risk of Down syndrome in a pregnancy is 1 in 4, or 25%. Although this risk assessment is likely to be greater than the patient’s age-related risk for Down syndrome, the chance that there is no Down syndrome in the pregnancy is still 3 in 4, or 75%.
The decision to pursue screening and/or diagnostic testing is individual, and patients may choose not to pursue any testing during their pregnancy. For patients who do choose screening, it is important to remember that perception of high and low risk for abnormalities varies, and the choice to pursue diagnostic testing is personal. Once you receive your result from a screening test, you may decide that your risk assessment is low enough that you are comfortable not undergoing diagnostic testing for chromosome abnormalities. Or, you may feel the risk assessment for the pregnancy is higher than you are comfortable with, and you would like diagnostic testing to determine whether or not a chromosome abnormality is actually present in the pregnancy.
The following questions may be helpful in deciding whether to undergo a diagnostic fetal test.
1. Is my new risk assessment higher or lower than my age-related risk?
2. Do I perceive my new risk number to be high or low?
3. If I choose not to pursue diagnostic testing, will I feel comfortable with my risk assessment throughout the course of my pregnancy?
4. How do I view my risk assessment in relation to the slight risks associated with diagnostic testing?
5.What would I do if I found out that the pregnancy is actually affected with a chromosome abnormality?
Our genetic counselors are available to discuss individual risk assessment with you and to help you make a personal decision about follow-up diagnostic testing.
