Genetic Screening
and Testing
Depending on your medical,
family and obstetrical histories, as well as upon your age, genetic
counseling and testing may be offered to you. Genetic counseling
may be as simple as a review of your family history and a few
specifically-targetted screening questions during your initial
obstetrical visit or as extensive as a consultation with trained
genetic counselors and perinatologists for amniocentesis or DNA carrier
testing.
The goal of all genetic
testing is to identify those babies who are at risk for medical
problems in pregnancy and after birth. People tend to think of
testing as only for Down Syndrome, but many disorders which occur
spontaneously or run in families can be diagnosed before birth.
Ultrasound
One of the simplest and easiest genetic screening tests is the
ultrasound. Ultrasound can be used to evaluate the physical characteristics
of the fetus which may help predict certain abnormalities. Measurements
of various body parts can be used to suggest an increased risk
of Down Syndrome, can look for congenital defects such as spina
bifida or leaky heart valves, and birth defects such as cleft
lip or club foot. Ultrasound is both an adjunct to other testing
such as amniocentesis, and a genetic screening test in its own
right. Ultrasound carries no risk of which we are aware, but does
not provide the highest levels of assurance desired by some parents,
because it relies on physical characteristics alone. Ultrasound
is also used for other evaluations of the fetus not involving
genetic screening. It may be used to check the position of the
fetus in the uterus, monitor fetal growth, locate the placenta,
evaluate the fluid around the baby or evaluate the fetus' behaviors
such as breathing, movement and muscle tone. Ultrasound is also
used to guide the physician during amniocentesis and chorionic
villus sampling (CVS). Here is an excellent site on ultrasound
during pregnancy,
with many images of the fetus as it develops.
Maternal Quad Screen
Another screening test
is the Maternal Quad Screen. The Maternal Quad Screen tests for
four chemicals in the mother's bloodstream :Alpha Fetoprotein
or AFP, Human Chorionic Gonadotropin (HCG), Inhibin and Estriol
. The test is performed at around the sixteenth week of pregnancy.
It originally consisted only of the AFP and was designed to screen
for defects in the baby's nervous system which allow spinal fluid
to leak out into the amniotic fluid - that is, open neural tube
defects such as spina bifida, open hydrocephaly and menigomyelocoel.
Since it was introduced, however, it has been found that the AFP
test can indicate other problems that may complicated the pregnancy.
In addition, by adding the results of the other three chemical
markers, more babies at risk for these problems may be discovered.
First of all, what is AFP?
It is a protein made by the fetus which can be detected in the
mother's blood, where it is called "maternal serum alpha
fetoprotein" (MSAFP). It is the MSAFP which can be measured
between 15.5 and 19 weeks. During this period, the amount of this
protein and the HCG, inhibin and estriol rise so rapidly that
normal amounts can be established for each week of pregnancy.
If the measurement of the Maternal
Quad Screen is above or below the limits of what is considered
normal for your length of pregnancy (as well as for your race,
weight and whether or not you are diabetic), this does not
mean that your baby is abnormal. It only means that you are a
candidate for further testing. In fact, 90% of babies whose mothers
have an abnormal triple screen are quite normal.
An abnormal quad screen
may indicate that the risk of your baby being born with trisomy
21 (or Down Syndrome) is as great as that of a woman aged 35;
it does not mean that your baby has Down Syndrome. You
may then be offered a very detailed ultrasound (Level II) or even
an amniocentesis in an effort to determine your baby's exact chromosomal
make-up and to rule-out Down Syndrome. Because the Quad screen
is not perfect at identifying those at risk of Down Syndrome,
a normal Quad screen does not guarantee that the baby does not
have Down Syndrome.
An elevated AFP indicates
that further testing would be in order to rule out a neural tube,
or spinal, defect. This test would be a level II ultrasound to
look for defects in the spinal cord, and possibly an amniocentesis
to check for the level of an enzyme which can indicate nervous
system problems.
In those pregnancies in
which the Quad screen is abnormal, but the followup testing shows
a normal baby, there may be a greater risk that the pregnancy
could be complicated by poor growh of the fetus, by preterm delivery,
by pregnancy-induced hypertension or toxemia. Therefore, if you
decide to have the Quad Screen, you may be able to obtain information
that allows you to take steps either to prevent or to reduce the
possibility of other complications.
This test is optional;
you may choose not to take the Quad Screen test, but wait until
your child is born to discover if any abnormality exists, and
thus avoid the emotional burden that some women feel while awaiting
the outcome of each level of more specific testing. It is also
important to remember that, if your care provider suggests additional
tests, he or she is only trying to gather the most information
possible to enable you to make informed decisions about your pregnancy.
Amniocentesis (or amnio) is a procedure usually
done at approximately 16 weeks gestation for genetic evaluation
of the fetal chromosomes. It is offerred to women who will be
35 years or older at the time of the birth, or to women with a
family history of a baby born with chromosomal problems. Amnio
may be done at other points in a pregnancy for other reasons,
but for genetic testing the second trimester is ideal. Using ultrasound
guidance, the doctor inserts a needle through the maternal abdomen
and into the uterus. A small amount of amniotic fluid is withdrawn.
The amniotic fluid contains cells from the baby's body, and these
are grown and the chromosomes are studied. The results of the
study are usually available in about 7 - 10 days. In addition
to the chromosome information, a protein called alpha feto protein
is measured in the amniotic fluid. This may help diagnosis a neural
tube defect such as spina bifida.
As with any medical procedure,
an amniocentesis carries some risks. The major risks are rupture
of membranes, bleeding and infection. These are rare, but if they
occur, it may cause loss of the pregnancy. Complications occur
in about 1 of every 200 amniocenteses.
Chorionic Villus Sampling
Some parents don't want to wait until 16 weeks for genetic testing.
A chorionic villus sample (CVS) can be done at an earlier time
in the pregnancy, usually around 11 weeks. CVS, like amniocentesis,
is offered to women who will be 35 or older at the time of the
birth, or with a family history of a child with a chromosomal
abnormality.The cells collected in the CVS procedure are from
the baby's placenta. A catheter is generally passed through the
mother's cervix, and some of the cells are withdrawn. Sometimes
a needle is passed through the mother's abdomen and cells are
withdrawn from the placenta. Results are generally ready in 3-4
days. CVS does not measure the amount of alpha feto protein in
the amniotic fluid, so if you want to know your risk of having
a baby with a neural tube defect, you should consider having a
blood test to measure the AFP. This is done between 15-18 weeks.
Like amniocentesis, CVS
is done under ultrasound guidance. The risk of ruptured membranes,
bleeding or infection is about 1 in 150. Both procedures are done
by the perinatologists at Yale-New Haven Hospital.
Remember, these tests are
optional. It is up to you and partner to decide whether you want
any screening or testing during your pregnancy. If you decide
that you would like to have the Maternal Quad Screen, we will
fill out the forms for you during an office visit when you are
about 15 weeks pregnant. If you decide to have an amniocentesis
or CVS, please contact Yale Genetics at 203-785-2661 to make an
appointment. Please let us know the date of your appointment so
that our billing staff may assist you in getting any pre-authorization
or pre-certification which your insurance company might require.
Obstetrics-Gynecology-Infertility
Group, PC
203-562-5181