Chorionic
Villus Sampling (CVS)
Who Should Consider CVS?
CVS should be considered by women age 35 or older
at the time of delivery, individuals who have had a child
with a chromosome abnormality, individuals who have a chromosome
translocation or other rearrangement, and couples at risk
for a prenatally diagnosable genetic disease (e.g. hemophilia
or cystic fibrosis). CVS may also be considered when certain
abnormalities are seen on ultrasound or when first trimester
screening has identified an increased risk for fetal anomalies.
CVS may not be appropriate for individuals with a family
history of neural tube defects (e.g., spina bifida or anencephaly).
When Is CVS Performed?
CVS is performed between 10-13 weeks after a
woman's last menstrual period (during the first trimester).
How Is CVS Performed?
There are two methods for obtaining chorionic villi. For many women, either
method can be safely performed. First, an ultrasound evaluation is performed
to locate the developing placenta and to date the pregnancy. Often, the placental
location determines which method of CVS is more appropriate. There are certain
other obstetrical considerations that may make one method preferable, including
uterine anatomy and vaginal infections.
- Transcervical CVS: A thin catheter (hollow
tube) is inserted through the vagina and cervix. Ultrasound
is used to guide the catheter to the edge of the developing
placenta. A small amount of placental tissue (chorionic
villi) is removed and sent to be analyzed.
- Transabdominal CVS: This procedure is
similar to amniocentesis.
Does The Procedure Hurt?
Most women feel the transcervical CVS is similar
to a pelvic exam. The transabdominal CVS is very similar
to an amniocentesis and some women do experience cramping
after the test. Both are described as mildly uncomfortable.
What Are The Risks After CVS?
Most miscarriages occur early in pregnancy regardless
of whether any invasive testing has been performed. A
test like an amniocentesis carries a risk for complications,
including miscarriage, of about 1 in 200 (0.5%). The
risk after a CVS procedure is slightly higher (about
1%). To help keep the risk as low as possible, women
are given precautionary instructions after the procedure.
In 1991, there were several reports of
babies with missing or shortened fingers or toes born
to women who had undergone CVS. These birth defects were
noted mainly when CVS was performed before the 10 th
week of pregnancy. As a result, CVS is now performed
only during or after the 10 th week of pregnancy and
all women being offered CVS are informed about the possible
small risk of limb defects.
Studies on the risk of limb defects following
CVS have had conflicting results. Some studies have found
no increased risk, while others suggest that CVS may
cause these limb abnormalities in about 1 in 3000 births.
However, recent studies suggest that CVS after 10 weeks
of pregnancy poses a small or no increased risk for these
limb defects. A study by the World Health Organization
in 1999 found no increased risk for these abnormalities
in babies born to more than 200,000 women who had CVS.
Based upon currently available information,
the following can be concluded:
- CVS is a relatively safe and accurate
procedure and is considered an acceptable alternative
to genetic amniocentesis.
- CVS is best performed at 10-13 weeks
of pregnancy based on the beginning of the last menstrual
period.
- Although it is uncertain whether there
is an increased risk of limb abnormalities following
CVS, it is important to understand that this controversy
has been raised and that the absolute risk may be in
the order of 1 in 3000 births. It is also important
to remember that 2-3% of all children are born with
a birth defect regardless of whether or not a woman
had prenatal diagnosis during her pregnancy.
What Is Analyzed From CVS?
The chorion is the outer membrane of the sac
that surrounds the fetus. Early in pregnancy, the chorion
is covered by fingerlike projections called villi. In
one area, the villi will implant in the uterus to develop
into the placenta and the remainder of the villi disappear.
The chorionic villi develop from the same fertilized
egg as the fetus so the genetic material should be the
same. The cells from the villi are cultured (grown) in
the laboratory. Chromosome analysis and other special
genetic testing can be performed on these cells.
What Can Be Detected Through CVS?
Chromosome abnormalities (such as Down syndrome)
can be detected through CVS. If indicated, some specific
genetic diseases can be diagnosed by DNA or enzyme analysis.
CVS cannot detect neural tube defects such as spina bifida.
Therefore, it is recommended that all women who undergo
CVS consider having their blood drawn at 15-18 weeks
of pregnancy for a spina bifida screening test (maternal
serum AFP assay). This can be arranged through your
obstetrician. Many women also wish to pursue a level
II ultrasound at 18-20 weeks for further information
regarding the physical development of the fetus. No method
of prenatal testing (CVS or amniocentesis) can guarantee
a baby will be born without birth defects or genetic
disease. Prenatal testing can only identify the diagnosable
problems for which a couple is known to be at risk. Unfortunately,
there are many conditions for which prenatal diagnosis
is not yet available.
How Accurate Are The Results from
CVS?
Chromosome results from CVS are greater than
99% accurate. Occasionally, results need to be clarified
through blood tests on the parents, ultrasound, a repeat
amniocentesis, or fetal blood sampling.
How Long Do The Results Take?
Chromosome results take approximately 2 weeks
for completion. Special studies for other genetic diseases
may take longer. Patients who elect to have an aneuploidy
FISH test on their sample can receive those results
in 24-48 hours from the day of the procedure.
