Chorionic Villus Testing (CVS)
Chorionic villus sampling (CVS) is the removal of a small piece of placenta
tissue (chorionic villi) from the uterus during early pregnancy to screen
the baby for genetic defects. CVS can be done through the cervix (transcervical)
or through the abdomen (transabdominal). The techniques are equally safe
when done by a provider with experience, although miscarriage rates are
slightly higher when done through the cervix. The health care provider
will use ultrasound to pick the safest approach and as a guide during sampling.
An ultrasound is performed to determine the position of the uterus, the
size of the gestational sac and the position of the placenta within the uterus.
The transcervical procedure is performed by inserting a thin plastic tube
through the vagina and cervix to reach the placenta. The provider uses
ultrasound images to help guide the tube into the appropriate area and
then removes a small sample of chorionic villus tissue.
The transabdominal procedure is performed by inserting a needle through
the abdomen and uterus and into the placenta. Ultrasound is used to help
guide the needle, and a small amount of tissue is drawn into the syringe.
The risks of CVS are only slightly higher than those of an amniocentesis.
Possible complications include:
- Rh incompatibility in the mother
- Rupture of membranes
Signs of complications include:
- Excessive bleeding
- Excessive vaginal discharge
Amniocentesis is a test during pregnancy that removes a small amount of
fluid from the sac around the baby. The doctor will find the exact location
of the baby, usually by performing an ultrasound.
An area of skin on the belly is cleansed. The doctor inserts a long, thin
needle through the abdomen and into the womb (uterus). A small amount
of fluid is taken from the fluid-filled sac that surrounds the baby and
sent to the lab for analysis.
Risks are minimal, but may include:
- Infection or injury to the baby
- Leaking of amniotic fluid
- Vaginal bleeding
Non Invasive Testing
Nuchal Translucency (NT)
The NT test uses ultrasound to measure the clear (translucent) space in
the tissue at the back of your developing baby's neck. Babies with
abnormalities tend to accumulate more fluid at the back of their necks
during the first trimester, causing this clear space to be larger than
average. The test will screen for chromosome abnormalities and, when combined
with blood work, neural tube defects.
The NT scan must be done when you're between 11 and 13.5 weeks pregnant.
It is often combined with blood work. The final results of the test are
available after the blood work is completed. There are many different
blood tests available with the NT test. The physician or genetic nurse
will discuss the options and help guide you to the best test for you.
The results will be given as a positive or negative test, and will give
you numbers concerning your risk of a chromosome problem. If the test
is positive showing an increased risk for a chromosome problem, further
testing will be offered to you. A thickened measurement may also indicate
a cardiac defect.
Remember that a normal screening result (screen negative) isn't a guarantee
that your baby has normal chromosomes, but it does suggest that a problem
is unlikely. Likewise, an abnormal screening result (screen positive)
doesn't mean that your baby has a chromosomal problem — just
that he's more likely to have one. In fact, most screen-positive babies
turn out not to have a problem.
First trimester screening - Combined
The blood screen measures two pregnancy related hormones: hCG and PAPP-A.
A sample of your blood is taken between 11 and 13 weeks of pregnancy,
at the same time the NT ultrasound scan is performed.
The substances in your blood that are markers of Down syndrome are pregnancy
associated plasma protein A (PAPP-A); and human chorionic gonadotropin (hCG).
The specific ultrasound marker of Down syndrome is nuchal translucency
In pregnancies with Down syndrome, PAPP-A tends to be low, and NT and hCG
tend to be raised.
The values of these markers are used together with your age to estimate
the risk of having a pregnancy affected with Down syndrome.
The combined accuracy rate for the screen to detect the chromosomal abnormalities
mentioned above is approximately 85 percent with a false positive rate
of 5 percent. This means that:
- Approximately 85 out of every 100 babies affected by the abnormalities
addressed by the screen will be identified.
- Approximately 5 percent of all normal pregnancies will receive a positive
result or an abnormal level.