CVS is a prenatal screening test that can provide very early diagnosis of fetal genetic or biochemical disorders. It involves extraction of a small amount of tissue from the villi of the chorion frondosum. This tissue is composed of rapidly proliferating trophoblastic cells that ultimately form the placenta. Although not a part of the fetus, these villus cells are genetically identical to the fetus and are considered fetal rather than maternal in origin. Some specialists advise that this procedure be reserved for evaluation of conditions that present relatively high genetic risks, such as hemoglobinopathies.

CVS differs from amniocentesis in several respects and therefore is not considered an alternative. With amniocentesis, the cells examined are desquamated fetal cells; the cells sampled in CVS divide rapidly and are easier to culture. For this reason, karyotyping (see Chapter 11) can be performed much more rapidly, and diagnostic information can be provided within 24 hours, much faster than with amniotic fluid cells. Also, CVS can be performed much earlier in pregnancy, typically at 7–11 gestational weeks, whereas amniocentesis usually is performed after 15 weeks’ gestation, with results available several weeks later. CVS, therefore, has the advantage of providing first-trimester diagnosis, which is of particular value when the choice is made to abort an affected fetus because first-trimester terminations of pregnancy are medically safer.

CVS reveals chromosome abnormalities and fetal metabolic or blood disorders. However, because CVS cannot be used to measure AFP, it cannot detect neural tube defects or other disorders associated with increased AFP levels.

Prenatal chromosomal microarray analysis (CMA) is a diagnostic test that can detect genetic abnormalities in a fetus using CVS cells or amniocytes. The sensitivity and specificity of prenatal CMA is much higher than the standard technique of karyotype analysis. It is possible to identify abnormalities such as DiGeorge syndrome, Miller–Dieker syndrome, telomere deletion syndromes, 22q11 syndrome, Smith–Magenis syndrome, Angelman syndrome, and Prader–Willi syndrome.

Indications for CVS include the following:

1. Abnormal ultrasound test

2. Fetus at risk for detectable mendelian disorders

   1. Tay-Sachs disease

   2. Hemoglobinopathies

   3. Cystic fibrosis

   4. Muscular dystrophy

3. Birth of previous child with evidence of chromosome abnormality

4. Parent with known structural chromosomal rearrangement

5. Diagnosis of fetal infection

1. Position the patient on her back to permit ultrasound documentation of the number of fetuses in utero and their viability and localization of trophoblastic tissue. Ask the patient either to maintain a full bladder or to empty the bladder so as to optimize the sampling path. A bimanual pelvic examination is often performed concurrently with this preliminary ultrasound examination.

2. Have the patient assume a lithotomy position. Insert a sterile speculum after the vagina has been cleansed with an iodine-based antiseptic.

3. Introduce a sterile flexible catheter with a stainless steel obturator into the vaginal canal and advance through the cervical canal into the trophoblastic tissue. The catheter is visually tracked by the ultrasound device.

4. Once the catheter is in place, attach a syringe to the end of the catheter to extract approximately 5 mL of tissue. Immediately examine the tissue sample under a low-power microscope to determine that both quantity and tissue quality are acceptable.

5. Make up to three passes of the catheter. Use a new, sterile catheter each time. After sufficient tissue has been gathered, use ultrasound again to monitor fetal viability. Use the tissue sample for chromosome and enzyme analysis and for other tests.

6. Be aware that a transabdominal method may also be used. This method is similar to amniocentesis except that the thin-walled needle is inserted into the chorionic bed.

Abnormal CVS results may indicate:

1. Abnormal fetal tissue

2. Chromosome abnormalities

3. Fetal metabolic and blood disorders

4. Fetal infection

Pretest Patient Care

1. Be aware that genetic counseling typically precedes any CVS procedure.

2. Explain the purpose, procedure, and risks of the test.

3. Ensure that a signed, witnessed informed consent document is obtained.

4. Have the patient drink four 8-oz glasses of water about 1 hour before the examination. The patient should not void until instructed to do so.

5. Obtain baseline measurements of maternal vital signs and FHR.

6. Advise the patient that she may experience cramping as the catheter passes through the cervical canal.

7. Help the patient to relax.

8. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Normal

• Negative for chromosomal and DNA abnormalities

• No fetal metabolic enzyme or blood disorders