Topic: Chorionic Villus Sampling and Amniocentesis Copy
In the United States, it is currently the standard of care to offer any pregnant woman a screening and diagnostic tests to assess for chromosomal conditions. In the first trimester the most commonly offered diagnostic test is chorionic villus sampling. Chorionic villus sampling can be accomplished via two routes 1) transcervical and 2) transabdominal.
Transcervical CVS is typically performed between ten and twelve weeks of pregnancy. A basic explanation to the patient would indicate that it is much like having a pap smear obtained in that a speculum is placed in the vagina, and after the cervix and vagina are cleansed, a small catheter is introduced under ultrasound guidance into the developing placenta. A portion of the placenta is “biopsied” and sent to the laboratory for evaluation. A “direct prep” preliminary answer can be available within twenty-four to forty-eight hours, whereas the culture result typically is available within seven to ten days.
Physicians also discuss with patients the availability of transabdominal chorionic villus sampling. This procedure can be offered as early as ten weeks and is available throughout gestation. This procedure is accomplished by cleaning off the patient’s abdomen and, under direct ultrasound guidance, directing a needle into the developing placenta and again aspirating or “biopsying” a portion of the placenta. The specimen is handled in much the same way that a transcervical specimen is. The physician should note, however, that transabdominal CVS is associated with a moderate degree of discomfort whereas the transcervical CVS is not. Prior to either chorionic villus sampling technique, the physician explains to the patient that there is a procedure related risk of approximately 1 in 455 of causing a pregnancy loss. This procedure related risk is additive to the “background risk”. The background risk depends upon a patient’s maternal age, but it is the risk that even if no procedure had been performed, the patient may have a spontaneous abortion (miscarriage). In comparison, the relative risk for amniocentesis is slightly less than CVS, at approximately 1 in 900. Chorionic villus sampling was developed as a diagnostic test for administration during the 10th to 12th weeks, since many patients desire to know early in their pregnancy whether the fetus has altered chromosomal patterns.
It should also be noted that these pregnancy loss risks have been calculated from experienced centers that perform a high volume of these procedures. The risk of pregnancy loss is affected by operator skill and experience.
Reference:
American College of Obstetricians and Gynecologists (2016, May). Practice bulletin 162: Prenatal diagnostic testing for genetic disorders. Washington, DC: American College of Obstetricians and Gynecologists.
Resources:
Explaining CVS tests to a patient: How Does CVS Work? by the Washington State Department of Health and the Genetic Support Foundation.
Explaining amniocentesis to a patient: How Certain Are Amniocentesis Results? by the Washington State Department of Health and the Genetic Support Foundation.