Chorionic Villus Sampling

Chorionic villus sampling takes a small sample of placental tissue, the chorionic villi, to obtain foetal chromosomal and genetic information. It is performed earlier in pregnancy than amniocentesis, usually between 10 and 13 weeks, which allows earlier diagnosis.

The sample is derived from the trophoblast, which is of foetal origin but may differ from the foetal tissues. This creates a risk of confined placental mosaicism, which requires follow up when it appears.

Approach. Sampling is transcervical, with a catheter passed through the cervix, or transabdominal, with a needle passed through the abdominal wall. Both use ultrasound imaging to direct the needle.

Usage. It is used in the same circumstances as amniocentesis, when an earlier result is preferred.

Indications. Abnormal screening, ultrasound anomaly, known parental chromosomal or single gene condition, and familial variant testing, when a result is needed in the first trimester.

Preparation and technique. Placental location is mapped by ultrasound. A sample of about 10 to 20 mg of villi is collected. The transabdominal route is selected where the placenta is anterior or where the cervix is unsuitable.

Contraindications and precautions. Active vaginal or cervical infection, cervical stenosis for the transcervical route, uterine anomaly that complicates catheter passage, and uncontrolled maternal bleeding. Rh negative women are offered anti-D immunoglobulin after the procedure.

Timing. Procedures performed before about 10 weeks are associated with a higher risk of limb reduction defects, so the procedure is generally postponed to 10 weeks or later.

Interpretation.

• Karyotype from cultured cells usually reports in 2 to 3 weeks.
• FISH and QF-PCR give rapid aneuploidy results, usually within 24 to 48 hours.
• Chromosomal microarray detects copy number variants, and variants of uncertain significance require counselling.
• Confined placental mosaicism shows an abnormality in the placenta that is absent from the foetal cells. This is not an automatic diagnosis of foetal abnormality, and amniocentesis is used to clarify it.

Causes of an uninterpretable result. Culture failure, insufficient tissue, and maternal cell contamination, which is more frequent with this sampling method than with amniocentesis.

Critical values and findings needing immediate action.

• A confirmed aneuploidy requires prompt counselling and clear discussion of options.
• Persistent mosaicism on a second sample requires specialist review.

Limitations and artefacts. Maternal cell contamination is a common cause of misleading results and is checked with a parental marker. Confined placental mosaicism can produce a result that differs from the foetal karyotype. Culture may fail in small samples.

Follow up tests. Amniocentesis to confirm foetal status when placental mosaicism is reported, detailed foetal ultrasound, and foetal echocardiography where a cardiac anomaly is present.

Complications. Pregnancy loss is the principal concern and is historically quoted as higher than that of amniocentesis, varying with operator experience and centre. Bleeding, leakage of fluid and infection are less common complications.

Special groups. Multiple pregnancy requires sampling of each sac. Women with uterine anomalies or cervical surgery usually have transabdominal sampling.