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Understanding Gestational Diabetes
Dr.Joe's Data Base
A Practical Guide to a Healthy Pregnancy
Approximately 3 to 5 percent of all pregnant women in the United States are
diagnosed as having gestational diabetes. These women and their families
have many questions about this disorder. Some of the most frequently asked
questions are:
* What is gestational diabetes and how did I get it?
* How does it differ from other kinds of diabetes?
* What can I do to control gestational diabetes?
* Will I need a special diet?
* Will gestational diabetes hurt my baby?
* Will I have diabetes in the future?
This hyperdocument, based on NIH Publication No. 93-2788, will address
these and many other questions about diet, exercise, measurement of blood
sugar levels, and general medical and obstetric care of women with
gestational diabetes. It must be emphasized that these are general
guidelines and only your health care professionals can tailor a program
specific to your needs. You should feel free to discuss any concerns you
have with your doctor or other health care provider, as no one knows more
about you and the condition of your pregnancy.
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What is gestational diabetes and what causes it?
Diabetes (actual name is diabetes mellitus) of any kind is a disorder that
prevents the body from using food properly. Normally, the body gets its
major source of energy from glucose, a simple sugar that comes from foods
high in simple carbohydrates (e.g., table sugar or other sweeteners such as
honey, molasses, jams, and jellies, soft drinks and cookies), or from the
breakdown of complex carbohydrates such as starches (e.g., bread, potatoes,
and pasta). After sugars and starches are digested in the stomach, they
enter the blood stream in the form of glucose (Figure 1). For the purpose
of this hypertext document the words sugar and glucose are used
synonymously.The glucose in the blood stream becomes a potential source of
energy for the entire body, similar to the way in which gasoline in a
service station pump is a potential source of energy for your car. But,
just as someone must pump the gas into the car, the body requires some
assistance to get glucose from the blood stream to the muscles and other
tissues of the body. In the body, that assistance comes from a hormone
(def) called insulin. Insulin is manufactured by the pancreas, a gland that
lies behind the stomach. Without insulin, glucose cannot get into the cells
of the body where it is used as fuel. Instead, glucose accumulates in the
blood to high levels and is excreted or "spilled" into the urine through
the kidney.
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Figure 1. Insulin: The Key to Turning Food into Energy
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When the pancreas of a child or young adult produces little or no insulin
we call this condition juvenile-onset diabetes or Type I diabetes
(insulin-dependent). This is not the type of diabetes you have. Unlike
women with Type I diabetes, women with gestational diabetes have plenty of
insulin. In fact, they usually have more insulin in their blood than women
who are not pregnant. However, the effect of their insulin is partially
blocked by a variety of other hormones made in the placenta (def), a
condition often called insulin resistance.
The placenta performs the task of supplying the growing fetus with
nutrients and water from the mother's circulation. It also produces a
variety of hormones vital to the preservation of the pregnancy. Ironically,
several of these hormones such as estrogen, cortisol, and human placental
lactogen (HPL) have a blocking effect on insulin, a "contra-insulin"
effect. This contra-insulin effect usually begins about midway (20 to 24
weeks) through pregnancy. The larger the placenta grows, the more of these
hormones are produced, and the greater the insulin resistance becomes. In
most women the pancreas is able to make additional insulin to overcome the
insulin resistance. When the pancreas makes all the insulin it can and
there still isn't enough to overcome the effect of the placenta's hormones,
gestational diabetes results. If we could somehow remove all the placenta's
hormones from the mother's blood, the condition would be remedied. This, in
fact, usually happens following a delivery.
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How does gestational diabetes differ from other types of diabetes?
There are several different types of diabetes. Gestational diabetes begins
during pregnancy and disappears following delivery. Another type is
referred to as juvenile-onset diabetes (in children) or Type I (in young
adults). These individuals usually develop their disease before age 20.
People with Type I diabetes must take insulin by injection every day.
Approximately 10 percent of all people with diabetes have Type I (also
called insulin-dependent diabetes).
Type II diabetes or noninsulin-dependent diabetes (formerly called
adult-onset diabetes) is also characterized by high blood sugar levels, but
these patients are often obese and usually lack the classic symptoms
(fatigue, thirst, frequent urination, and sudden weight loss) associated
with Type I diabetes. Many of these individuals can control their blood
sugar levels by following a careful diet and exercise program, by losing
excess weight, or by taking oral medication. Some, but not all, need
insulin. People with Type II diabetes account for roughly 90 percent of all
diabetes.
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Who is at risk for developing gestational diabetes and how is it detected?
Any woman might develop gestational diabetes during pregnancy. Some of the
factors associated with women who have an increased risk are obesity; a
family history of diabetes; having given birth previously to a very large
infant; a still birth, or a child with a birth defect; or having too much
amniotic fluid (polyhydramnios). Also, women who are older than 25 are at
greater risk than younger individuals. Although a history of sugar in the
urine is often included in the list of risk factors, this is not a reliable
indicator or who will develop diabetes during pregnancy. Some pregnant
women with perfectly normal blood sugar levels will occasionally have sugar
detected in their urine. The Council on Diabetes in Pregnancy of the
American Diabetes Association strongly recommends that all pregnant women
be screened for gestational diabetes. Several methods of screening exist.
The most common is the 50-gram glucose screening test. No special
preparation is necessary for this test, and there is no need to fast before
the test. The test is performed by giving 50 grams of a glucose drink and
then measuring the blood sugar level 1-hour later. A woman with a blood
sugar level of less than 140 milligrams per deciliter (mg/dl) at 1-hour is
presumed not to have gestational diabetes and requires no further testing.
If the blood sugar level is greater than 140 mg/dl the test is considered
abnormal or "positive." Not all women with a positive screening test have
diabetes. Consequently, a 3-hour glucose tolerance must be performed to
establish the diagnosis of gestational diabetes.
If your physician determines that you should take the complete 3-hour
glucose tolerance test, you will be asked to follow some special
instructions in preparation for the test. For 3 days before the test, eat a
diet that contains at least 150 grams of carbohydrates each day. This can
be accomplished by including one cup of pasta, two servings of fruit, four
slices of bread, and three glasses of milk every day. For 10 to 14 hours
before the test you should not eat and not drink anything but water. The
test is usually done in the morning in your physician's office or in a
laboratory. First, a blood sample will be drawn to measure your fasting
blood sugar level. Then you will be asked to drink a full bottle of glucose
drink (100 grams). This glucose drink is extremely sweet and occasionally
makes some people feel nauseated. Finally, blood samples will be drawn
every hour for 3 hours after the glucose drink has been consumed. The
normal values for this test are shown in Table 1.
Table 1. 3-Hour Glucose Tolerance Test for Gestational Diabetes
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If two or more of your blood sugar levels are higher than the diagnostic
criteria, you have gestational diabetes. This testing is usually performed
at the end of the second trimester or the beginning of the third trimester
(between the 24th and 28th weeks of pregnancy) when insulin resistance
usually begins. If you had gestational diabetes in a previous pregnancy or
there is some reason why your physician is unusually concerned about your
risk of developing gestational diabetes, you may be asked to take the 50
gram glucose screening test as early as the first trimester (before the
13th week). Remember, merely having sugar in your urine or even having an
abnormal blood sugar on the 50-gram glucose screening does not necessarily
mean you have gestational diabetes. The 3-hour glucose tolerance test must
be abnormal before the diagnosis is made.
How does gestational diabetes affect pregnancy and will it hurt my baby?
The complications of gestational diabetes are manageable and preventable.
The key to prevention is careful control of blood sugar levels just as soon
as the diagnosis of gestational diabetes is made.
You should be reassured that there are certain things gestational diabetes
does not usually cause. Unlike Type I diabetes, gestational diabetes does
not generally cause birth defects. For the most part, birth defects
originate sometime during the first trimester (before the 13th week) of
pregnancy. The insulin resistance from the contra-insulin hormones produced
by the placenta (def) does not usually occur until approximately the 24th
week. Therefore, women with gestational diabetes generally have normal
blood sugar levels during the critical first trimester (def).
One of the major problems a woman with gestational diabetes faces is a
condition the baby may develop called "macrosomia." Macrosomia means "large
body" and refers to a baby that is considerably larger than normal. All of
the nutrients the fetus receives come directly from the mother's blood
(figure 2). If the maternal blood has too much glucose, the pancreas of the
fetus senses the high glucose levels and produces more insulin in an
attempt to use this glucose. The fetus converts the extra glucose to fat
(def). Even when the mother has gestational diabetes, the fetus is able to
produce all the insulin in needs. The combination of high blood glucose
levels from the mother and high insulin levels in the fetus results in
large deposits of fat which causes the fetus to grow excessively large, a
condition known as macrosomia. Occasionally, the baby grows too large to be
delivered through the vagina and a cesarean delivery may become necessary.
The obstetrician can often determine if the fetus is macrosomic by doing a
physical examination. However, in many cases a special test called
ultrasound is used to measure the size of the fetus. This and other special
tests will be discussed later.
In addition to macrosomia, gestational diabetes increases the risk of
hypoglycemia (low blood sugar) in the baby immediately after delivery. This
problem occurs if the mother's blood sugar levels have been consistently
high causing the fetus to have a high level of insulin in its circulation.
After delivery the baby continues to have a high insulin level, but it no
longer has the high level of sugar from its mother, resulting in the
newborn's blood sugar level becoming very low. Your baby's blood sugar
level will be checked in the newborn nursery and if the level is too low,
it may be necessary to give the baby glucose intravenously. Infants of
mothers with gestational diabetes are also vulnerable to several other
chemical imbalances such as low serum calcium and low serum magnesium
levels.
All of these are manageable and preventable problems. The key to prevention
is careful control of blood sugar levels in the mother just as soon as the
diagnosis of gestational diabetes is made. By maintaining normal blood
sugar levels, it is less likely that a fetus will develop macrosomia,
hypoglycemia, or other chemical abnormalities.