Articles
Register for An Edited Excerpt from "Your Developing Baby"
Chapter 13
Peter M. Doubilet, MD, PhD
Professor of Radiology, Harvard Medical School
Carol B. Benson, MD
Professor of Radiology, Harvard Medical School
Roanne Weisman
Tests for the Baby's Health: Amniocentesis and Others
Ultrasound lets the doctor check on some aspects of the baby's health,
especially those that involve the baby's structure and size. There are other
tests that can be used together with ultrasound to get extra information about
the baby. Some of these tests involve the pregnancy support systems that we
discussed in the previous chapter: amniocentesis, which removes amniotic fluid
for testing; chorionic villus sampling (CVS), which obtains small pieces
of the placenta for evaluation; and umbilical blood sampling, which
involves removing a sample of the baby's blood to be analyzed. The most
frequently used of these procedures is amniocentesis. These tests are performed
only if there are certain indications that the baby might be at risk and that
the test can provide the doctor with information to help care for the mother and
baby. Such indications include the age of the mother (thirty-five or older), a
family history of or a previous child with hereditary disease, or an ultrasound
finding that raises concern for the health of the baby.
Amniocentesis
Stay away from that baby! Under the watchful "eye" of the ultrasound image (the
grey area in this diagram), the doctor makes sure that the amniocentesis needle
stays away from the baby and the umbilical cord.
Amniocentesis in a sixteen-week pregnancy. Ultrasound monitoring helps the doctor make sure the needle avoids the baby and umbilical cord.
Amniocentesis is the removal of a sample of amniotic fluid from the
gestational sac through a thin needle. This test can be used to check for
hereditary or genetic diseases--such as Down syndrome, an abnormality of the
baby's chromosomes--that may not show up on ultrasound alone. It also has other
uses in special circumstances, such as to check for infection in the amniotic
fluid that might require treatment, or to determine whether the baby's lungs are
ready to work on their own to breathe air.
You might wonder how the fluid that the baby is floating in can give us
information about the baby's chromosomes. The answer lies in the behavior of
cells. Every time you touch or brush your skin against something, some of your
skin cells fall off. These cells are microscopic, of course, so you don't see
them. The same thing happens to the baby. Even though the baby is floating in
amniotic fluid, skin cells are always falling off and floating around in the
fluid. By removing a small amount of amniotic fluid, we get a sample of the
baby's cells and put these cells through a genetic analysis to find out about
the baby's chromosomal and genetic makeup.
Amniocentesis is not a new procedure; it has been used for about fifty years,
well before the common use of ultrasound. But today, thanks to ultrasound,
amniocentesis is safer than ever. As you can see in the diagrams and image
above, a needle is inserted into the gestational sac and a syringe is used to
withdraw a small amount of fluid--about four teaspoons for a genetic diagnosis,
and considerably less for other testing. Because the baby is moving around in
that same fluid, continuous ultrasound guidance is used to make sure that the
needle does not poke the baby. If the baby approaches the needle, this will be
seen on the ultrasound image and the doctor can move the needle away from the
baby.
During this relatively painless procedure, about one-half to one and a half
inches of the needle extends into the amniotic fluid, which is then drawn up
into a syringe. When the right amount of fluid has been withdrawn, the needle is
removed, and the uterine wall closes up tightly over the tiny hole so that there
is no bleeding or leakage of fluid.
Chorionic Villus Sampling
Chorionic villus sampling, commonly referred to as CVS, is another way to test
for certain hereditary or genetic abnormalities when there are indications that
the baby might be at risk. It is important to realize that even if a doctor
recommends a CVS or amniocentesis, in the vast majority of cases the results of
these tests turn out to be normal and the baby is healthy.
When there are indications for an amniocentesis or CVS for genetic testing, how
do the doctor and mother choose between these two tests? Amniocentesis, a
quicker and simpler procedure than CVS, is chosen more often than CVS. But CVS
provides answers at an earlier point in pregnancy than amniocentesis. CVS is
usually done between ten and twelve weeks' gestation, and the results are
available in just a few days, while amniocentesis is commonly done at fifteen to
eighteen weeks' gestation and the results come back one to two weeks later. So
CVS is often chosen if the mother or doctor doesn't want to wait until after
fifteen weeks to get the answer.
A Genetic Blueprint
As we learned in earlier chapters, the placenta is made up of cells that come
from both mother and baby. The chorion is the part of the placenta that comes
from the baby, so its cells have the same genetic makeup as the baby. By
extracting some cells--called chorionic villi (plural of villus)--from the
placenta, growing them in the laboratory, and looking at their chromosomes, we
can see a genetic blueprint of the baby and determine whether there are genetic
abnormalities. There are two ways to remove the chorionic villus cells, shown in
the pair of diagrams below.
Transabdominal CVS. This method involves inserting a needle through the
mother's abdomen into the placenta, guided by ultrasound. Once the needle is
in the proper location, tiny pieces of placenta are removed by the suction of a
syringe attached to the needle.
Transcervical CVS. This method uses a soft catheter inserted into the
vagina and through the cervix, and then threaded--using ultrasound
guidance--along the outside of the gestational sac until the tip is inserted
into the placenta. Then small bits of placenta are drawn out into the
syringe.
Chorionic villus sampling. The shaded areas below the ultrasound transducer on the diagrams show the region seen in the ultrasound image. The transabdominal approach (left diagram) uses a needle inserted through the mother's abdomen and the wall of the uterus into the placenta. The transcervical approach (right diagram) uses a flexible catheter inserted through the mother's vagina and cervix, with the tip advanced into the placenta.
Following either CVS procedure, a technician takes the contents of the
syringe and uses a special technique to separate the baby's cells (chorionic
villi) from the mother's cells (decidua). The baby's cells are then genetically
analyzed.
The decision about whether to use the transabdominal or transcervical technique
depends, in part, on the location of the placenta. If the placenta is in the
front of the uterus, a transabdominal CVS is generally best; if it is toward the
mother's back, the transcervical technique is often used. Whichever technique is
chosen, the test is relatively painless for the mother and the needle or
catheter does not enter the gestational sac or touch the baby.
Chorionic Villus Sampling in Real Life
The next ultrasound images and accompanying diagrams show you what the
doctor inserting the needle or catheter sees on the ultrasound screen when
performing chorionic villus sampling. The video and diagram below show a
transabdominal CVS. You can see the needle as a straight white line ending up in
the placenta. Ultrasound guidance makes sure that the needle is in the placenta
and does not enter into the amniotic fluid or come into contact with the baby.
Transabdominal chorionic villus sampling in a twelve-week pregnancy.
The image and diagram below the insertion of a catheter--seen as a curved white line--for a transcervical CVS, again using ultrasound guidance to make sure that the procedure is safe and successful.
Transcervical chorionic villus sampling in a twelve-week pregnancy.
Umbilical Blood Sampling
Doctors often use blood tests to check on the health of a patient. An
obstetrician caring for a pregnant woman will often do blood tests on the
mother, such as a test in early pregnancy to confirm that she is in fact
pregnant, or a test for blood sugar levels to check for diabetes. Occasionally,
the obstetrician needs to examine the baby's blood to make sure that the baby is
healthy. This might be done to check whether the baby is anemic or to test for
the presence of enzymes in the baby's blood that might indicate a health
concern.
When the doctor needs to do a blood test on the baby, blood is obtained from the
umbilical vein--which is located inside the umbilical cord and transports blood
from the placenta to the baby. Using ultrasound guidance, a tiny needle is
passed through the mother's abdomen into the point at which the umbilical cord
attaches to the placenta. This is a good spot because the cord is anchored there
and does not move. A few drops of blood are pulled into a syringe attached to
the needle. There is no pain to the baby and very little to the mother, since
the placenta and umbilical cord do not have nerves to feel pain.
In rare circumstances, a similar procedure can be done to treat the baby when a
problem is present. For example, if the baby is anemic the doctor can give the
baby a transfusion by injecting blood into the umbilical vein. In other cases,
medication can be injected.
The two drawings below show the positions of the ultrasound transducer, needle,
syringe, and umbilical cord when umbilical blood sampling is done. The path of
the needle depends on where the placenta is located and where the umbilical cord
attaches to it. If the placenta is in the front of the uterus (left diagram), the needle
goes through the placenta into the umbilical vein without going through the
amniotic fluid. If the placenta is in the back, toward the mother's spine (right
diagram),
the needle must go through the amniotic fluid to get to the umbilical vein.
During these procedures, the doctor uses ultrasound to guide the needle into the
proper location and to avoid any contact with the baby. Whether the placenta is
in the front or the back, the needle enters the umbilical cord where it is
anchored to the placenta.
Umbilical blood sampling
The remaining images and diagrams show the umbilical blood sampling procedure in real life, with the placenta in different locations.
Umbilical blood sampling when the placenta is in the front of the uterus.
Umbilical blood sampling when the placenta is in the back of the uterus.
