Diabetes & Glucose Tolerance Testing (GTT) by
What is gestational diabetes?
Gestational diabetes (GD) simply
means elevated blood sugar during pregnancy.
To understand it, you must first understand
the normal changes in pregnancy metabolism
(34). When you are pregnant, certain hormones
make your insulin less effective at transporting
glucose, the body’s fuel, out of your
bloodstream into your cells. This increases
the amount of circulating glucose, making it
available to your baby for growth and development.
This “insulin resistance” increases
as pregnancy advances. As a result, your blood
glucose levels after eating rise linearly throughout
pregnancy. By the third trimester, you will
tend to have higher blood glucose levels after
eating than nonpregnant women (hyperglycemia),
despite secreting normal and above normal amounts
of insulin. During overnight sleep, the excess
insulin has a chance to mop up, which causes
morning glucose levels to be lower on average
than in nonpregnant women (hypoglycemia).
In the 1950s, some researchers
wondered whether sugar values at the high end
of the range for pregnancy would predict the
development of diabetes later in life. They
tracked a population of women and in 1964,
they reported that, yes, it did (40). The extra
stress of pregnancy revealed a woman’s
“prediabetic” status. This shouldn’t
have surprised anyone, because high-weight
women are much more likely to have higher glucose
values in pregnancy than average-weight women
and to eventually develop diabetes. However,
doctors knew diabetes posed grave threats to
the unborn baby, so they worried that glucose
levels that were high, but not in the diabetic
range, might also do harm. This concern launched
what eventually became an avalanche of studies
that ended by defining a whole new category
of pregnancy complication called “gestational
diabetes,” although “glucose intolerance
of pregnancy” would be a more accurate
description. Those studies, and their premise,
were fundamentally flawed.
Is gestational diabetes a health
The theory that GD could have
the same adverse effects of diabetes was faulty
on its face, because GD does not share the
risk factors of either type of true diabetes.
In Type I diabetes, extremes of low and high
blood sugar early in pregnancy can cause malformations
or miscarriage. GD women make normal or above-normal
amounts of insulin and have normal blood-sugar
metabolism in the first trimester (22). Either
Type I or II, long-standing diabetes can damage
maternal blood vessels and kidneys, causing
hypertension or kidney complications. These
can in turn jeopardize the fetus. Gestational
diabetics do not have long-standing diabetes.
The one problem GD shares with both types is
that chronic hyperglycemia can overfeed the
fetus, resulting in a big baby. This is generally
defined as a birth weight of more than 8 lbs.
13 oz. (4,000 grams) or a birth weight in the
upper ten percent for length of pregnancy (large
for gestational age).
Theory aside, the studies designed
to test it had significant weaknesses. They
included women who were known diabetics prior
to pregnancy. They selected women for glucose
testing based on such risk factors as prior
stillbirth, current hypertension, or extreme
overweight, indications that alone could explain
poorer outcomes (12). They failed to account
for compounding factors, such as that glucose
intolerance associates with increasing maternal
weight and age, which themselves strongly predict
large babies and maternal hypertension. Finally,
they used management protocols that increased
risks such as starvation diets, early induction
and withholding nourishment from the newborn
(18). Despite these flaws, researchers concluded
that mildly deviant glucose values in pregnancy
caused serious harm.
We now know that GD doesn’t
increase the risk of stillbirth or congenital
malformations (4). A couple of modern studies
have concluded otherwise, but they didn’t
take into account that women with high blood
sugar are more likely to have other risk factors
for poor outcome, or that some women had undiagnosed
diabetes prior to pregnancy (17,24). Indeed,
the fact that these studies were of women whose
blood sugar had been normalized by treatment
proves that GD is not the culprit. Besides,
GD testing and treatment could not affect the
incidence of congenital malformations under
any circumstances, because testing isn’t
done until the third trimester. By that time,
the baby is long since fully formed.
We also know that maternal glucose
level correlates poorly with birth weight.
While GD somewhat increases the odds of having
a baby weighing in the upper ten percent (16,36),
most of this results from GD’s association
with other factors, in particular, maternal
Other supposed risks of GD are
preeclampsia, glucose intolerance in the child
and childhood obesity. As before, GD is only
found in company with these complications;
it doesn’t cause them. For example, studies
show that blood glucose level plays little
if any role in high-weight children compared
with maternal weight before pregnancy (8,25).
Also, as before, normalizing blood sugar fails
to prevent these problems, which absolves GD
All this being said, there is
a needle in the haystack. About one in a thousand
pregnant women tested will have sugar values
in the range of true diabetes (2). These women
may have been diabetic before pregnancy and
not known it, or pregnancy may have been enough
of a metabolic stress to tip them into diabetes.
These women may benefit from being identified
How do practitioners test for
Testing for GD is a two-stage
process. The first step is a screening test,
which is generally administered to all pregnant
women. The screening test is usually given
somewhere between week 24 and 28. For this
test, you may be asked to drink a glucose solution
and have a blood sample drawn an hour later,
or you may simply be asked to give a blood
sample. If your blood glucose value exceeds
a threshold amount, you will be asked to return
for an Oral Glucose Tolerance Test (OGTT).
The various protocols disagree on the amount
of glucose and the threshold value (29).
For the OGTT, you will be asked
to come in after fasting overnight. Blood will
be drawn, you will be given a glucose solution
to drink, and blood will be drawn one, two
and three hours later. The glucose solution
may make you nauseous. As with the screening
test, the recommended amount of glucose and
the diagnostic thresholds vary from protocol
to protocol (29). Some guidelines only stipulate
a fasting glucose and a two hour value (29).
What are the problems with gestational
A diagnostic test should be reproducible,
meaning you get the same results when you repeat
the test. Thresholds should be values at which
complications either first appear or incidence
greatly increases; and normal ranges should
apply to the population being tested. The OGTT
is none of the above.
Obstetricians adopted data from
the original 1950s studies as the normative
curve for all pregnant women, but they shouldn’t
have. For one thing, those researchers tested
women without regard to length of gestation,
whereas today, doctors typically test women
at the beginning of the third trimester. Glucose
values rise linearly throughout pregnancy,
but no corrections have been made for this
(15). For another, they studied a population
that was sixty percent white and forty percent
black. Hispanics, Native Americans and Asian
women average higher blood sugars than black
or white women (10,57). This means values for
that 1950s population have been established
as norms for all women, which in turn means
that some women are being identified as diseased
simply because of race.
The OGTT also isn’t reliable.
When pregnant women undergo two OGTTs a week
or so apart, individual test results disagree
twenty to twenty-five percent of the time (5,23).
A person's blood sugar values after ingesting
glucose (or food) vary widely depending on
many factors. For this reason, the OGTT has
been abandoned as a diagnostic test for true
diabetes in favor of high fasting glucose values,
which show much greater consistency, or values
after eating of 200 mg/dl or more, which are
rare (46,52). Moreover, pregnancy compounds
problems with reproducibility. Because glucose
levels rise linearly throughout pregnancy,
a woman could “pass” a test in
gestational week 24 and “fail”
it in week 28 (55). These same reproducibility
problems hold true for the glucose screening
test that precedes the OGTT (47,55).
More importantly, no threshold
has ever been demonstrated for onset or marked
increase in fetal complications below levels
diagnostic of true diabetes. The original researchers
chose their cutoffs for convenience in follow-up,
but all studies since have used their criteria
or some modification thereof as a threshold
for pathology in the current pregnancy. Numerous
studies since have documented that birth weights
and other outcomes fail to correlate with the
1950s or anybody else's thresholds. Today’s
researchers acknowledge that the risks of glucose
intolerance almost certainly form a continuum
and that screening and diagnostic thresholds
are arbitrary (7,29-30,48,51).
Several organizational bodies
that have looked critically at the GD research
have come out against GD testing. A Guide to
Effective Care in Pregnancy and Childbirth,
the bible of evidence-based care, relegates
screening for gestational diabetes to “Forms
of Care Unlikely to be Beneficial (12).”
The American College of Obstetricians and Gynecologists
says no data support the benefits of screening
(1). The U.S. Preventative Services Task Force
and the Canadian Task Force on the Periodic
Health Examination both conclude that there
is insufficient evidence to justify universal
GD screening (4,11).
How is gestational diabetes treated?
The main elements of GD treatment
Normalizing blood sugar: The first
step is a diet low in sugars and
carbohydrates. Some diets also limit
calories. If diet fails to control
blood glucose levels, insulin injections
Monitoring blood sugar: In most cases
this will mean pricking your finger
and testing your blood once, and
more commonly, several times a day.
Many protocols include:
Monitoring fetal well-being: Many
practitioners order repeated fetal
surveillance tests beginning at or
before the due date. The most common
is the nonstress test, which looks
at the fetal heart rate changes in
response to fetal movements or Braxton-Hicks
contractions (normal, nonlabor tightening
of the uterus).
Ultrasound scan to estimate fetal
Planned delivery: This may be either
induction of labor or elective cesarean
section. Induction is often at, or
sometimes before, the due date.
Monitoring newborn blood sugar: Some
protocols call for checking the baby’s
blood sugar, which involves a heel
What are the problems with gestational
The two questions asked of any
therapy are: “Is it safe?” and
“Is it effective?” GD management
GD treatment per se has never
been shown to have benefits. In fact, it is
virtually untested. The first and only random
assignment trial, the standard for determining
care because this design eliminates many sources
of bias and ensures similar groups, was published
in 1997. It concluded that intensive treatment
offered no advantages over advising women to
eat healthy (16). Meanwhile, several studies
have found that identification as a gestational
diabetic in and of itself substantially increases
the odds of cesarean section (3,19,38,50).
Individual components of GD protocols
also fail the safety/effectiveness test:
Diet or diet plus insulin therapy:
The standard GD diet is a healthy diet. However,
while it reduces blood glucose to normal range
in most women, it has little or no effect on
birth weight (54). Many women, though, are
prescribed limited calorie diets. Reducing
calorie intake by more than one-third causes
the body to switch to a starvation metabolism
(ketosis) that produces byproducts known to
be harmful to the baby (31). Limiting food
intake can also lead to malnutrition (27).
Aggressive insulin use can cause underweight
babies and symptomatic episodes of low blood
sugar (hypoglycemia) (3,32). A Guide to Effective
Care in Pregnancy and Childbirth lists both
diet treatment and diet plus insulin treatment
under “Forms of Care Unlikely to be Beneficial
Tests of fetal well-being: Of
the four random assignment trials of nonstress
testing, the most commonly used fetal surveillance
test, none found any benefit for testing, although
they were in populations of women at moderate
to high risk (41). All tests of well-being
have high false-positive rates, meaning the
test says there is a problem when there isn’t.
This leads to unnecessary inductions and cesareans
with all their attendant risks.
Fetal weight estimates: Ultrasound
predictions that the baby will weigh over 4,000
grams are wrong one-third to one-half of the
time (6,9,14,20,33,56). As with fetal well-being
tests, the belief that the baby is big leads
to unnecessary inductions and cesareans. Two
studies showed that when obstetricians believed,
based on ultrasound, that women were carrying
babies weighing over 4,000 grams, half had
cesareans, versus less than one-third of women
not thought to have babies this big, but who
actually did (35,56).
Induction of labor or planned
cesarean: Many doctors induce labor in the
belief it averts cesareans due to big babies.
Some think induction or planned cesarean prevents
shoulder dystocia (the head is born, but the
shoulders hang up). Studies of induction and
planned cesarean for suspected big baby show
no benefits for either practice (6,9,14,20,33,49,56).
Monitoring newborn blood sugar:
The reasoning behind this is that if the mother
has high blood-sugar levels, the baby will
produce extra insulin. After birth, this excess
insulin can cause low blood sugar. No studies
have tested whether checking the blood sugar
of a baby who shows no symptoms of low blood
sugar has any value. However, test results
can lead to the baby being given a bottle of
sugar water or formula, which interferes with
establishing breastfeeding, separation from
the mother for observation in the nursery,
Finally, treatment also fails to prevent increased
incidence of preeclampsia, impaired glucose
tolerance in children, and childhood overweight
Another rationale given for diagnosing
and treating gestational diabetics is identifying
women at risk for developing Type II diabetes.
However, predicting who is likely to develop
diabetes can be done equally well on the basis
of race, ethnicity, and weight.
Curiously, while several prominent
GD researchers and experts acknowledge the
lack of sound data supporting their recommendations,
none have backed off (1,26,37,39). These experts
devise GD guidelines for practicing doctors
and midwives, most of whom have no idea how
shaky the GD edifice is. Even those who doubt
the value of screening all or most women for
GD may have little choice if testing and treatment
is the community standard of care.
How does diagnosis as a gestational
diabetic affect your pregnancy and birth?
The standard GD diet is a good
one; adequate calories, limit simple sugars,
moderate fat intake, eat whole grains and plenty
of fruits and vegetables and eat smaller meals
more frequently. Also beneficial is the advice
to engage in moderate, regular exercise. If
that was all that happened, identification
as a gestational diabetic would be a good thing.
Some tracking of blood sugars to make sure
they aren’t drifting into the true diabetic
range is probably also a good thing, as is
identifying the one in a thousand women who
has or will develop glucose values in that
range. However, most women will find themselves
caught up in frequent doctor visits, multiple
daily blood tests, restrictive diets, possibly
insulin injections, repeated fetal surveillance
tests and a considerable chance of a labor
induction or cesarean section.
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