Chorionic villus sampling (CVS) is a procedure used to diagnose certain birth defects in the first
trimester of pregnancy. It involves inserting a small catheter (tube) through
the cervix, under ultrasound guidance. A small piece of the placenta (the chorionic villi) is then removed and sent to the lab for analysis. Both the placental and fetal tissues originate from
the same cell line and are theoretically genetically identical.
Thus, by obtaining a tiny sample of the chorionic villi from the
placenta, the laboratory can detect Down syndrome, other
chromosome abnormalities and certain genetic conditions when a family history exists of such a disease.
CVS is performed between ten and twelve weeks
from the first day of the last menstrual period. Couples who may wish to consider CVS include:
- Women 35 years of age and older
- Parents who have had a child with Down syndrome or other
chromosome abnormality
- Couples who are known carriers of a chromosome rearrangement
- Couples who have a family history of a genetic condition
for which testing is available
Until recently, amniocentesis was the accepted
mainstay of prenatal diagnosis. Amniocentesis involves inserting
a needle into the amniotic sac surrounding the fetus. Amniocentesis
is performed after the 14th week gestation and test results take
two weeks. One advantage of CVS over amniocentesis is that test
results following CVS may be available within the first trimester
of pregnancy. For most couples, this means earlier reassurance.
The potential disadvantages of CVS are complications such as vaginal bleeding or cramping occur more
frequently following CVS than amniocentesis. These risks, including miscarriage, are estimated to occur in less than 1 in 125 procedures. The American College of Obstetricians and Gynocologists
(ACOG) has recently made a statement based on recent research
indicating that the risks associated with CVS may significantly less and infact no more than
the amniocentesis when performed by a Maternal Fetal Medicine
specialist such as our physicians. It is importnat to realize, however, that
many miscarriages occur naturally in the first trimester, even
without CVS.
Certain defects of the extremities have been reported
in infants whose mothers underwent CVS. This type of limb deficiency
is known as transverse limb defects and involve the absence of
the distal structures of the limb (those furthest from the trunk).
It has been hypothesized that these defects may be caused by a
disruption of the vascular system of the limb. The overall risk
for transverse limb defects following CVS is approximately 0.03%-0.10%
(1/3,000-1/1,000). The risk and severity of the defect seems to
be related to the timing of the procedure, with procedures performed
before nine weeks associated with limb defects more frequently. This risk of limb defects has NOT been seen when the procedure is performed bettwen 10-12 weeks.
Genetic counseling prior to the procedure is recommended
so a couple can make an informed decision. During the counseling
session the CVS procedure will be described along with
its potential risks and benefits.
If a couple decides to pursue CVS, a follow-up ultrasound
examination should be performed between 18 and 20 weeks of pregnancy
to evaluate fetal growth and anatomy. Some obstetricians will also offer isolated maternal
serum alpha fetoprotein (msAFP) screening between the 15th and
18th week of pregnancy. This is a blood test
which screens for neural tube defects such as open spina bifida. Amniotic fluid AFP levels are evaluated when
an amniocentesis is performed, and therefore msAFP screening is not necessary
following an amniocentesis.
Please click HERE
for a diagram of an CVS procedure.
Please click HERE
for a Brochure on CVS.
