Prenatal diagnostic testing procedures
A genetic counseling appointment is scheduled for any women considering a diagnostic prenatal procedure, such as amniocentesis, in order to review the benefits, risks and limitations of the procedure. Available genetic testing options are also reviewed during this session. The different possible procedures are:
Many genetic tests and other testing options exist for women undergoing a prenatal procedure. Genetic counselors review these options before your procedure is performed. They will help you understand the benefits and limitations of these tests and help you decide which are right for you and your family.
Chromosomes are the structures in our cells that contain our genetic information. Routine chromosome analysis involves looking at the chromosomes of a baby by analyzing cells from the amniotic fluid (obtained from amniocentesis), placenta (obtained from CVS) or other fetal tissue. This analysis evaluates for missing or extra genetic information, which can lead to genetic syndromes, such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome), caused by an extra copy of either chromosome 21, 18 or 13, respectively. Chromosome analysis results are typically available within two weeks. The chance for having a baby with a chromosomal trisomy increases as women get older, but it can occur at any age. For example, at age 28 a woman has approximately a 1 in 1,000 risk for Down syndrome compared to a 1 in 100 risk at age 40.
Chromosomal microarray analysis
Chromosomal microarray analysis (CMA) is the most comprehensive prenatal genetic testing option and is performed in conjunction with chromosome analysis. CMA has the ability to detect more than 180 genetic conditions, including most of the conditions that can be found on a routine chromosome analysis, but adds many more that would be missed by a chromosome analysis. The risk for most of these conditions does not depend on a woman’s age. If one combines the risk for having a baby with any of these conditions, it is often higher than the risk for having a baby with Down syndrome, especially when the ultrasound shows that the fetus has one or more birth defects or is not growing well. The results of CMA can be available in 7 days.
Fluorescence in situ hybridization
Fluorescence in situ hybridization (FISH) is a more rapid way to test for common chromosome problems such as Down syndrome, trisomy 18, trisomy 13 and Turner syndrome. These results typically take 48 hours. Confirmatory chromosome analysis or CMA is always performed in addition to FISH testing.
Neural tube defects
Spina bifida is an abnormality of the neural tube and occurs in 1 in 1,000 babies. Many neural tube defects, such as spina bifida, can be evaluated during pregnancy by analyzing amniotic fluid via an amniocentesis. Ultrasound and blood work can also be helpful tools in detecting spina bifida during pregnancy. Neural tube defects cannot be evaluated with other prenatal procedures (i.e.CVS).
Other specific testing
Testing for a specific genetic condition may be indicated depending on family history or pregnancy history. A genetic counselor will meet with you prior to your procedure to determine whether additional testing is indicated for your pregnancy.