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 10^th to
12^th 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.