Fetal Screening - Diagnostic & Screening Tests

DIAGNOSTIC & SCREENING TESTS

Prenatal testing is available for many important birth defects. These tests may be considered either "diagnostic" or "screening" tests. Diagnostic tests can tell whether a fetus has a particular condition with certainty but may provide limited information about other types of birth defects. Also, some diagnostic tests such as amniocentesis carry a small risk of complications that could result in fetal death. Screening tests are not as accurate as diagnostic tests but do help to identify patients at risk who might benefit from a diagnostic test. Screening tests will result in both "false positive" results and some missed cases. A primary advantage of screening tests is they are not invasive and so do not carry a risk to the fetus. When optimally performed and interpreted by experts, ultrasound can both "screen" for a variety of major defects and can also make a specific diagnosis. For example, a quality ultrasound with fetal anatomic survey should be able to identify virtually all cases of spina bifida and neural tube defects, whereas AFP testing would be considered a screening test. For other types of defects - particularly Down syndrome - screening tests such as ultrasound may not be diagnostic themselves but can help identify those patients at highest risk who might consider genetic amniocentesis.

The following is a partial list of the types of conditions in which screening and diagnostic tests are available.

SCREENING TESTS	DIAGNOSTIC TESTS
DOWN SYNDROME TESTING
Maternal Age Combined Screen for fetal aneuploidy (11-14 weeks, ideally 11-12 weeks) Biochemical Testing First or Second Trimesters (11-14,15-20 weeks Integrated Screen (First trimester screen + 2nd trimester screen with one result) Sequential or Step wise screen (First trimester screen + 2nd trimester screens with two results). Second trimester genetic sonogram (15-20 weeks)	Genetic amniocentesis (14-20 weeks) Chorionic villus sampling (CVS) (10-13 weeks)
SPINA BIFIDA & NEURAL TUBE DEFECTS
AFP screening (80-90% detection rate). Level 1 office ultrasound (in reality, virtually all spinal defects should be detected or suspected on any fetal screening survey)	Targeted or level 2 ultrasound Amniocentesis with AFP and acetylcholinesterase testing (nearly diagnostic)
CLEFT LIP / PALATE
A carefully performed level 1 office ultrasound can detect or suspect some cases of cleft lip/ palate	Detailed ultrasound with fetal survey may be diagnostic and a screening test.
OTHER STRUCTURAL BIRTH DEFECTS
Level 1 office ultrasound can detect or suspect many major structural birth defects	Detailed ultrasound with fetal survey can detect the majority of important birth defects and in many cases is also diagnostic.
TAY SACHS DISEASE
Blood tests can determine whether the parents carry the genetic mutation responsible for Tay-Sachs Disease. A Tay-Sachs carrier has one normal gene for hex A and one Tay-Sachs gene. The carrier does not have the illness and leads a normal, healthy and full life. However, when two carriers become parents, there is a one-in-four chance that any child they have will inherit a Tay-Sachs gene from each parent and have the disease. In that situation, diagnostic tests may be performed on the fetus.	Detailed ultrasound with fetal survey can detect the majority of important birth defects and in many cases is also diagnostic.
OTHER JEWISH RELATED CONDITIONS
Blood tests can screen for other conditions that are more common among Jewish patients as part of a "Jewish" panel	Amniocentesis or CVS may be diagnostic for some of these conditions
SICKLE CELL DISEASE
Blood tests on the parents will identify people who have either sickle cell trait or a form of the disease, as well as a number of other less common inherited hemoglobin abnormalities. If both parents have the sickle cell trait, the fetus will have sickle cell disease in 1/4 of cases and this can be tested for as a diagnostic test.	If both parents are carriers of Sickle cell, the fetus will have Sickle cell disease in 1/4 of cases. This can be tested for prenatally to determine for certain whether the fetus is affected.
THALASSEMIA
When two carriers become parents, there is a one-in-four chance that any child they have will inherit a thalassemia gene from each parent and have a severe form of the disease. There is a two-in-four chance that the child will inherit one of each kind of gene and become a carrier like its parents; and a one-in-four chance that the child will inherit two normal genes from its parents and be completely free of the disease or carrier state. These odds are the same for each pregnancy when both parents are carriers. If both parents are carriers, then diagnostic tests may be performed on the fetus to see if it is affected.	If both parents are carriers of thalassemia, the fetus will have thalassemia in 1/4 of cases. This can be tested for prenatally using chorionic villus sampling (CVS) or amniocentesis. Early diagnosis is important so that treatment can prevent as many complications as possible.
CYSTIC FIBROSIS
The American College of Obstetricians and Gynecologists (ACOG) recommends that screening tests be available to all couples who are planning pregnancy or who are pregnant. . Testing identifies the most common mutations associated with CF. A genetic mutation is carried by 1 in 29 Caucasians compared to 1 in 46 for those of Hispanic background, 1 in 65 for African Americans, and 1 in 90 for Asian Americans. Both the mother and father will carry a CF mutation in about 1 of 800 pregnancies among Caucasians. Only if both parents are carries is the fetus at risk for CF, and this will occur in 1/4 of cases when both parents are carriers. So, even though a genetic defect is relatively common among Caucasians, CF affects only about 1 in 3200 Caucasian pregnancies. Some screening protocols test the mother first and the test the father only if the mother carries one of the known mutations, whereas other screening protocols test both the mother and father at the same time. A simple saliva test is often used as the screening test.	Amniocentesis or CVS can be used to identify whether a fetus is affected. However, parents should know this is not performed as part of a standard genetic amniocentesis, but can be performed in families known to be at increased risk. Following birth, the sweat chloride test is a standard test..
FRAGILE X SYNDROME
About one in 250 women in this country carries the fragile X gene. A woman is considered a carrier of fragile X syndrome if she has either a pre-mutation (60 to 200 trinucleotide repeats within her FMR-1 gene) or a full mutation (greater than 200 trinucleotide repeats within her FMR-1 gene). This can be determined by a blood test. Approximately one-third to one-half of female carriers of a full mutation will exhibit signs of fragile X syndrome, but are a full usually less severely affected than males with full mutations. but are usually less severely affected than males with full mutations. a full mutation will exhibit signs of fragile X syndrome, but are usually less severely affected than males with full mutations.	Amniocentesis or CVS can be used to identify the mutation. However, parents should know this is not performed as part of a standard genetic amniocentesis, but rather only in families known to be at increased risk. Also, a positive test cannot always determine whether a baby will be mentally retarded. While almost all boys who inherit the full mutation have mental retardation or serious learning disabilities, only about one-third to one-half of affected girls will have mental retardation or will have serious learning disabilities, only about one-third to one-half of affected girls will have mental retardation or serious learning disabilities.