Prenatal Testing-Chorionic Villi

BACKGROUND

Chorionic villus sampling (CVS) is the removal of a small sample of chorionic (placental) tissue for prenatal diagnosis, usually by aspiration biopsy with a thin plastic catheter inserted transcervically or transabdominally. The procedure is usually performed at 11 weeks of pregnancy, and therefore can provide results earlier than amniocentesis.

INDICATIONS FOR TESTING

Advanced maternal age (35 years or older at time of delivery)
Abnormal ultrasound
Known chromosomal abnormality in the parent (translocation etc.)
Family history of malformative/mental retardation disorders
Family history of disease/syndrome which could be detected by molecular or biochemical methods (see molecular outline)
Previous child with chromosomal abnormality

SAMPLE REQUIREMENTS

Separate 5-20 mg of cleaned villi are placed in a conical centrifuge tube containing transport media. Place one branch of villus (for direct harvest) in a small vial containing colcemid. The transport media and the vials are available from the laboratory.

Specimens sampled more than 3 days previously, frozen, or less than 3 mg are not acceptable.

SPECIMEN HANDLING

Forward villi to the laboratory the same day if preliminary results on direct harvest are desired. If it is impossible to deliver villi the same day, store overnight in the transport media in the refrigerator, or ship in a Styrofoam container by overnight courier.

PROCEDURE

Cytogenetic analysis is accomplished by direct and culture techniques whenever possible. The direct technique takes advantage of active division of the cytotrophoblast cells in the outer layer of the villi. The dividing cells are arrested by colcemid during transport, and results are available 24-48 hours after receipt. In the culture method the villi are desegregated by enzymatic methods, and the resulting suspension is used to establish primary cultures. The mesenchymal cells of the villous core released by this procedure can be used for cytogenetic analysis 6-10 days after initiation of cultures.

Five cells and one karyotype are prepared from the direct method, and additional 20 cells and two karyotypes are prepared from the culture method.

INTERPRETATION

CVS will detect fetal sex, and any gross abnormalities of the sex chromosomes or autosomes such as trisomies, monosomies, translocations, deletions and duplications. Deletions and rearrangements on the molecular level cannot be seen. Molecular and biochemical testing can also be done on fresh or cultured villus cells. Alpha-fetoprotein testing cannot be performed on this tissue and requires alternative diagnostic options for the detection of neural tube defects (e.g. fetal ultrasound, amniotic fluid). Occasionally chromosomal mosaicism may be found in CVS samples. This necessitates amniocentesis for confirmation of the results.

RESULTS

Results are presented according to the international standards for chromosomal nomenclature (ISCN), and a full explanation of the karyotype is provided. Any abnormal or variant results are followed by genetic counseling.

Molecular, biochemical and FISH testing can also be done on fresh or cultured cells. Alpha-fetoprotein testing can be performed for detection of neural tube defects.

Prenatal Testing-Chorionic Villi