Invasive prenatal diagnosis allows vital details to be obtained regarding the fetus that can include genetic, biochemical, or physiologic data. This chapter describes amniocentesis, chorionic villus sampling (CVS), and fetal blood sampling (FBS), which are the most frequently used invasive techniques to obtain this information. Most perinatal centers have the technical expertise to perform all of these procedures and to select the best method for each clinical scenario. However, certain conditions meriting specialized techniques may require referral to a fetal center.
To complement diagnostic imaging and invasive fetal procedures, perinatal centers must provide detailed genetic counseling. With this, available clinical options are described, their risks are explained, and their consequences are communicated in a nondirective, supportive way.
Invasive Prenatal Testing Counseling
Myriad tests for prenatal screening are available to the practicing obstetrician. The American College of Obstetricians and Gynecologists (2016b) recommends that all pregnant women regardless of age be offered aneuploidy screening before 20 weeks' gestation. Diagnostic testing is then offered to women whose screening test results are positive. In that same publication, the College also endorsed that all pregnant women, regardless of age, should be counseled regarding the option of invasive or diagnostic fetal testing after a thorough explanation of the differences between screening and diagnostic tests.
Invasive prenatal tests yield diagnostic results but have the potential for procedure-related complications. Therefore, pretest counseling by the obstetrician or maternal-fetal medicine (MFM) specialist or by a genetic counselor or medical geneticist is recommended. Elements of counseling are listed in Table 13-1.
TABLE 13-1.Elements of Counseling before Invasive Prenatal Testing |Favorite Table|Download (.pdf) TABLE 13-1. Elements of Counseling before Invasive Prenatal Testing
Indication for the test
Risks/potential complications of procedure
Alternative therapies or natural history of condition if expectant management elected
Specific test(s) ordered
Diagnostic accuracy and test limitations
Gestational age range for which an invasive test can be safely performed
Waiting interval before a final result is expected
Postprocedural activity limitations
Prior to the procedure, the patient is ideally instructed regarding suggested postprocedural activity restrictions. Further, a description of how complications will be evaluated is recommended.
Prior to any invasive test, several procedural steps are recommended. Needed specimen containers are assembled and are prelabeled to reduce errors. Laboratory requisitions are similarly premarked to reflect the procedure and type of specimen.
A "time out" is suggested, given the risk of pregnancy complications including fetal loss. Elements that are reviewed and confirmed include correct patient identity and medical record number, the planned procedure, tests to be performed from the samples, potential intraprocedural complications, correct specimen labeling, and pertinent maternal laboratory results. The latter may include maternal blood type, Rh status, and positive results from hepatitis B, hepatitis C, ...