Perhaps you or someone you know is having a baby. One of
the most important questions (or worries) during a pregnancy
is "Is the baby healthy?" or "Is the baby developing
properly?" After all, the uterus in which the baby grows is
essentially a "black box" relative to the outside world. How
do we know that a baby is developing properly throughout the
40 weeks of gestation?
In this edition of How Stuff
Works, we will examine the tests that
obstetricians (the doctors who care for pregnant women
during pregnancy and delivery) use to assess the development
of an unborn child.
Photo courtesy of Philips
examination during pregnancy is one common form of
Pregnancy Involves Many Tests
course of a pregnancy, an expecting mother will have many
tests. They fall into the following categories:
Let's look at the
various tests to see how they work and what they can tell us
about the developing baby.
- pregnancy test - the first test
- routine, non-invasive tests - these occur during
each visit to the obstetrician's office
- blood pressure
- urine glucose/protein
- fetal heartbeat
- blood sampling tests - usually done once at
various times during pregnancy
- blood type, Rh Factor
- determine iron levels
- HIV, Hepatitis B, syphilis
- glucose tolerance test
- triple screen test
- swab tests - usually done once at various times
- Pap smear - to check for STDs, various bacteria
(varies according to the laws of each state)
- Group B streptococcus screening
- done at least one time during pregnancy
- tests done under special circumstances
- chorionic villus sampling
- fetal blood sampling or percutaneous umbilical blood
- fetal monitoring
The Pregnancy Test
This test is usually the
first test conducted when you suspect that you may be
pregnant. There are a variety of home testing kits available
over-the-counter and all detect a protein hormone called
human chorionic gonadotropin (hCG). When an egg is
fertilized, the embryo begins to produce hCG. Levels of hCG
increase after conception and can be detected in the mother's
urine. By 10 days after conception, hCG levels are about 25
milli-International Units (mIU).
Typically, the home test is a urine test for hCG:
If a positive test
occurs, you generally call your doctor and a second test is
performed at the office to confirm the pregnancy. The doctor
may also order a blood test to determine the precise quantity
of hCG present, which can be used to assess the baby's health.
- You collect a sample of urine. You would usually
use the first urine in the morning, when hCG levels are the
most concentrated, or wave the test wand through the
- If you collected the urine, you can either dip the test
wand into the cup or place a drop on the test wand.
- The test wands or dipsticks have a plastic coating
embedded with antibodies
- The test wands also have a second antibody to hCG
linked with some color tag (e.g., colored latex beads,
that produces a color reaction).
- If sufficient levels of hCG are present in the
urine (more than 25 mIU), then the hCG will bind with the
second antibody and cause a color reaction to occur
(i.e., a positive test result).
Routine Non-Invasive Tests
These tests are
done every time you visit your obstetrician and include:
- blood pressure
- urine glucose
- urine protein
- fetal heartbeat - beginning when the baby's heart
is developed enough to to be heard
The increased blood volume
and fetal blood circulation that occurs in pregnancy increases
the demands on your cardiovascular system, especially your heart. So,
your blood pressure will be measured regularly to detect any
signs of high blood pressure or pregnancy-induced
hypertension. About five percent of pregnant women
experience pregnancy-induced hypertension starting about the
20th week of pregnancy. This condition can can lead to the
hypertension, along with swelling (edema) and protein
in the urine (albuminuria), comprise a condition known
as pre-eclampsia. The cause of pre-eclampsia is unknown
and the treatment is premature delivery of the baby, if
possible. Sometimes, high doses of magnesium sulfate can be
given to delay the symptoms until the baby can be delivered
safely; no one knows why this treatment can work.
- Pre-term labor
- Separation of the placenta, leading to bleeding
- Reduced kidney
function or failure
- Reduced blood flow to the baby, which can retard its
growth and development
Your blood pressure will be measured with a blood
pressure gauge or sphygmomanometer (read this
question about blood pressure gauges for more details).
doctor's visit, you will be asked to pass a test strip through
your urine stream or collect a sample of urine, which will be
tested with a strip that measures the amount of glucose in
your urine. The presence of glucose in the urine is an
indication of gestational diabetes, a form of diabetes
that usually develops around the 20th week of pregnancy.
Gestational diabetes causes the following complications:
can be treated usually by monitoring the mother's diet.
However, sometimes the mother must take insulin to control her
blood glucose levels. Gestational diabetes in the mother
usually goes away once the baby has been delivered.
- The baby grows larger than normal and develops more fat.
Large babies are difficult to deliver.
- The baby's pancreas must secrete large amounts of
insulin to get rid of the excess sugar coming from the
mother. After birth, when the baby is no longer receiving
these high amounts of sugar from the mother, the high
insulin levels can cause the baby's blood sugar to fall
dangerously low (i.e., hypoglycemia).
- Some babies from mothers with gestational diabetes have
trouble breathing when they are delivered (i.e.,
The test strip contains two enzymes (glucose oxidase
and peroxidase), a chemical (orthotolidine) and
a yellow dye impregnated in the paper. The reactions go like
If no glucose is present, then the test
strip remains yellow. If glucose is present, then the color
can vary from light green to dark blue, depending upon the
concentration of glucose in the urine.
- Glucose oxidase converts glucose into gluconic
acid and hydrogen peroxide.
- Peroxidase reacts the hydrogen peroxidase with
orthotolidine to produce a blue color.
- The yellow dye spreads the color change out over
a wider range in proportion to the amount of glucose
presence of protein in the urine indicates a problem in kidney
function and is one of the symptoms of pre-eclampsia, as
mentioned above. To detect protein in the urine, the test
strip has a pH buffer (citrate buffer) and a color
indicator (bromphnol blue) impregnated in the paper. At
the normal pH of the paper, most of the indicator is not
ionized. Proteins can bind to the nonionized form and release
hydrogen ions, which changes the pH and the color of the
paper. If protein is present, then the color of the paper will
change from yellow to green or blue, depending upon the
concentration of protein.
the more emotional times in an early pregnancy may be the
first time you hear the baby's heartbeat. The baby's heartbeat
can be seen in a Doppler
ultrasound as early as five to six weeks of development.
By 12 to 13 weeks, your doctor can hear the heartbeat using a
specialized ultrasound stethoscope or Doppler
stethoscope. The Doppler stethoscope works like a regular
machine except that it does not give an image. Instead,
the echoes are counted and the count is displayed on a LCD readout.
If the stethoscope has a speaker, you can hear the baby's
amplified heart beat.
Blood Sampling Tests
In the first trimester
of pregnancy, you may have blood drawn to determine the
The hCG, viral antibody, syphilis
and blood type tests look for and/or quantify the amount of antibodies
to these substance in the blood. The iron test directly
measures the iron by burning a sample in a high temperature
flame and quantifying the amount of light given off at the wavelength of
light specific for iron, which is related to the amount of
- hCG levels (discussed above)
- Blood iron content - important for hemoglobin
sufficient oxygen to the growing baby
- Blood type and Rh compatibility - assess problems
between mother and baby's blood types (see Rh
Blood Testing for details)
- Presence of antibodies to viruses
(e.g., HIV, hepatitis B) or presence of bacteria
Between week 25 to 28 gestation, you will have
a glucose screening test for gestational diabetes. You will
drink a soda containing high amounts of glucose, or sugar, and
will have your blood drawn one hour later. Blood glucose will
be measured by a glucose oxidase reaction described in the previous
section. If the glucose level is high, you may be asked to
take an additional glucose-tolerance test. In this
test, you drink a high-glucose solution on an empty stomach
and blood samples will be taken at regular intervals (usually
every hour) for three hours. Blood glucose levels will be
measured again. The timecourse information from a glucose
tolerance test is a better indication of your body's response
to a glucose load to diagnose gestational diabetes.
DefectsThe baby's brain
and spinal cord develop from the outer layer of the
embryo called the ectoderm. The ectoderm dimples
inward along the long axis of the embryo, the two edges
come together, the edges seal and the folded portion
pinches off to form a tube (neural tube). The
nervous system, including the brain and spinal cord,
develop from the neural tube. If the neural tube does
not close, then a condition called spina bifida
develops and the baby's brain and spinal cord do not
develop properly, resulting in mental handicaps and even
death. Neural tube defects can be prevented by
supplementing the mother's diet with folic
acid (a major component of prenatal vitamins).
Triple Screen Test
test is done in the second trimester and measures three
AFP is produced by the baby and
makes its way into the mother's blood. Typically, AFP levels
are low. However, high levels of AFP indicate that the baby's
neural tube has failed to close (i.e., a neural tube
defect). An ultrasound
examination may then be done to confirm this finding.
- alpha-fetoprotein (AFP)
At this time in development, hCG levels in
combination with AFP levels can tell us whether the baby has
any abnormalities in the number of chromosomes. A high level
of hCG in combination with a low level of AFP suggests a
chromosomal abnormality. The most common disorder of this type
is Down syndrome (extra chromosome #23); see Human
Chromosomal Abnormalities for more details. If the level
of hCG is high and no fetal heartbeat is detected, then there
may be a molar pregnancy. A molar pregnancy is when a
piece of tissue, usually leftover from a previous pregnancy,
grows rapidly, destroys the baby and forms a benign tumor. An
ultrasound examination may then be done to confirm this
Estriol is a hormone made by the baby's adrenal
glands. The levels of estriol in the mother's blood indicate
the health of the baby. If the estriol levels drop, then the
baby may be threatened and need to be delivered. Low levels of
estriol may also indicate Down syndrome or neural tube defect.
At different times during
pregnancy, your doctor may take swab samples (Q-tip) from your
vagina or rectum. The cells on the swab will be examined under
a microscope or cultured to determine various conditions or
In the first trimester, you may have a Pap smear in
which cells from your cervix are examined to look for signs of
cervical cancer. Later, when you are closer to delivery
(around week 35), your doctor will take swabs from your vagina
and rectum. These swabs will be cultured to look for Group
B streptococcus bacteria. These bacteria are common in
many women and are considered a normal part of an adult
woman's reproductive system. For an unborn child, however,
these bacteria can spread during delivery and cause several
Group B strep infections can be
treated with IV antibiotics before delivery.
- respiratory distress - problems with breathing that
threaten the baby's life
- mental retardation
- vision problems
- hearing loss
You may have one or more
ultrasound tests during your pregnancy. The first test may be
done within the first four weeks to date the pregnancy and
determine the due date. This is done by measuring the baby's
crown to rump length. Accurate charts are available to
determine the age of the fetus from these measurements.
By about weeks 11 to 13, ultrasound may be used to examine
various aspects of your baby's developments, such as:
weeks, you may be able to determine the sex of the baby by
observing its sex organs with ultrasound. For more details on
ultrasound, see How
- Is the neural tube closed?
- Are the internal organs developing properly?
- Does the heart beat OK, and at what rate?
Tests Done Under Special Circumstances
problems with the baby's development are detected by other
means or the parents have risk factors for various genetic
diseases, special tests may be required. For example, if the
mother is age 34 or older, she has an increased risk of having
a baby with Down syndrome. So, the parents, in consultation
with the obstetrician, may elect to sample fluid or tissue
from the baby to determine its genetic make-up. These samples
can be obtained through three procedures:
In these tests, the doctor uses
a needle or suction tube to sample fluids or the baby's tissue
with the aid of ultrasound to see what he/she is doing. The
tissue/fluid samples are analyzed in a laboratory to determine
the number of chromosomes (karyotype) and other
biochemical tests (e.g., AFP). The results from the tests can
be used to make decisions about possibly terminating the
pregnancy or preparing the parents to deal with any genetic
defects (genetic counseling).
- chorionic villus sampling
- fetal blood sampling
- fetal monitoring
Amniocentesis is a test
in which the doctor samples the fluid (amniotic fluid)
that surrounds the growing baby in the uterus. Floating in the
amniotic fluid, there are cells from the baby as well as
fluids from the baby (urine). The cells can be grown and the
fluid analyzed for various biochemical markers.
Amniocentesis is usually done between 15 to 18 weeks and
for mothers with two or more risk factors (e.g., older than
age 34, and/or family history of Down syndrome or other
genetic diseases). If you were to have this test done, you
would go into an examination room with your doctor, an
ultrasound technician, and your partner or other family
member. The procedure goes like this:
It takes about
two to three weeks before the results of the amniocentesis can
- The technician swabs your abdomen with an antiseptic
- The technician will use ultrasound to determine where
the baby is in your uterus and where there are pockets of
fluid away from the baby.
- Once these areas are located, your doctor will insert a
needle through your abdomen and into the uterus. The doctor
constantly monitors the ultrasound to avoid poking the baby.
- The doctor withdraws about 30 to 60 ml of amniotic
fluid, which the baby will replace within a day.
- The doctor places this fluid in one or more sterile
cups, marks the cups, and sends them off to a laboratory.
- The doctor removes the needle and places a bandage over
- You may have another ultrasound exam to assess the
baby's health after the procedure.
Diagram showing the sampling and
analysis procedures of amniocentesis
Amniocentesis has a quoted risk of 0.5 percent, meaning
that 1 in 200 procedures have some type of complication (e.g.,
infection, miscarriage or poking the baby with the needle). In
most cases, these percentages are much lower, especially since
ultrasound has been used to guide the doctor.
Like amniocentesis, chorionic villus
sampling (CVS) is done to obtain a sample of the baby's
tissue for determining genetic disorders. In contrast, CVS
obtains tissue from the placenta (chorion) instead of
fluid. Because the chorion is derived from the baby, not the
mother, it bears the genetic characteristics of the baby. The
procedure is similar to amniocentesis except that the tissue
can be taken by inserting a needle through the abdomen or a
sample tube through the cervix.
Diagram showing the sampling and
analysis procedures of chorionic villus
Like amniocentesis, CVS is done when the mother has one or
more risk factors for genetic diseases. CVS can be done
earlier than amniocentesis, usually in the latter part of the
first trimester ( between weeks nine and 11). The results of
CVS can be obtained sooner than amniocentesis because no
culturing of the tissue is required. However, because CVS only
gets tissue, some of the biochemical tests done with
amniocentesis cannot be done with CVS. Also, CVS carries a
higher risk (about 1 percent), mostly because it is a newer
procedure relative to amniocentesis.
Fetal blood sampling can be done
from 18 weeks to full term. In this procedure, fetal blood is
obtained from the umbilical cord for analysis. Like
amniocentesis, the doctor (aided by ultrasound) inserts a
needle through the mother's abdomen into the umbilical cord
and withdraws a blood sample. The blood sample is sent to a
lab for analysis. The genetic results from fetal blood
sampling can be obtained much faster than with amniocentesis
because no tissue culturing is involved. In addition, this
technique can be used to transfuse the fetus with compatible
blood in the event that the baby's and mother's Rh factors do
not match. The overall risk of the fetal blood sampling
procedure is 0.5 - 1 percent.
monitoring is usually done in the third trimester (for
high-risk pregnancies, pre-term labor or overdue babies) and
during delivery. This test involves strapping an electronic
monitor to the mother's abdomen that measures the electrical
activities of the following:
movements inside the uterus can be assessed and correlated
with its heart rate. There are two types of test done:
- fetal heart beat
- mother's heart beat
- mother's uterine muscle contractions
- Non-stress test - the baby's heart rate should
increase when the baby moves (about 15 beats for 15 seconds
at least twice in a 20 minute period).
- Stress test - the baby's heart rate should
increase when the uterus contracts. Uterine contractions are
caused by infusing a medication (Pitocin) or by
stimulating the mother's nipples.
These tests are used by obstetricians to determine how well
the baby will handle the stress of delivery.
As you can see, there are a wide variety of ways to measure
your baby's growth and development before it is born. Prenatal
testing is completely normal during pregnancy and can often
detect problems early, before they progress.
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