What is
gestational diabetes? By
Henci Goer.com
Article taken from ParentsPlace.com
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 (hyperglycaemia),
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 RISK?
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 foetus. Gestational
diabetics do not have long-standing diabetes.
The one problem GD shares with both types is that
chronic hyperglycaemia can over feed the foetus,
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 weight
(10, 13, 21, 28, 43, 57).
Other supposed risks of GD are
pre-eclampsia, 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 (42, 44-45, 53).
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 and treated.
HOW DO PRACTITIONERS TEST FOR GESTATIONAL DIABETES?
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 DIABETES TESTING?
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 OGTT's 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 favour
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 foetal complications below levels
diagnostic of true diabetes. The original researchers
chose their cut offs 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 Gynaecologists 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 are:
- 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
are prescribed.
- 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 foetal well-being:
Many practitioners order repeated foetal surveillance
tests beginning at or before the due date. The
most common is the nonstress test, which looks
at the foetal heart rate changes in response
to foetal movements or Braxton-Hicks contractions
(normal, nonlabour tightening of the uterus).
- Ultrasound scan to estimate
foetal weight.
- Planned delivery: This may
be either induction of labour or elective caesarean
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 stick.
WHAT ARE THE PROBLEMS WITH GESTATIONAL DIABETES TREATMENT?
The two questions asked of any therapy are: “Is
it safe?” and “Is it effective?”
GD management is neither.
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
caesarean 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 by-products 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 (hypoglycaemia)
(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 (12).”
- Tests of foetal well-being:
Of the four random assignment trials of nonstress
testing, the most commonly used foetal 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 caesareans
with all their attendant risks.
- Foetal 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 foetal
well-being tests, the belief that the baby is
big leads to unnecessary inductions and caesareans.
Two studies showed that when obstetricians believed,
based on ultrasound, that women were carrying
babies weighing over 4,000 grams, half had caesareans,
versus less than one-third of women not thought
to have babies this big, but who actually did
(35,56).
- Induction of labour or planned
caesarean: Many doctors induce labour in the
belief it averts caesareans due to big babies.
Some think induction or planned caesarean prevents
shoulder dystocia (the head is born, but the
shoulders hang up). Studies of induction and
planned caesarean 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, or both.
Finally, treatment also fails
to prevent increased incidence of pre-eclampsia,
impaired glucose tolerance in children, and childhood
overweight (42,44-45,53).
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 foetal
surveillance tests and a considerable chance of
a labour induction or caesarean section.
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