at delivery. Most women with gestational diabetes will
not become hyperglycemic during labor, because they
are not eating (although they are generally allowed to
drink fluids). We often provide an intravenous infusion of 5% dextrose to meet the caloric needs of labor.
If maternal glucose concentrations exceed 120 mg/dL a
constant intravenous insulin infusion can be administered starting at 1 U/h. This is virtually always needed
for gravidas with type 1 diabetes, sometimes needed for
those with type 2 diabetes, and rarely necessary for gestational diabetes.
Once delivery has occurred, and the fetal–placental
unit is no longer releasing hormones that cause insulin resistance, maternal glucose metabolism generally rapidly returns to normal. Because some
women with gestational diabetes actually had undiagnosed preexisting diabetes before their pregnancy, we
measure a fasting plasma glucose on the morning after
delivery to make sure that no further treatment is
needed at that time.
Obstetric Management
TESTS OF FETAL WELL-BEING
Pregnancies complicated by gestational diabetes are at
increased risk of stillbirth (2). Although there is no
single best evidence-based approach to monitoring fetal well-being in gestational diabetic pregnancies, the
ACOG has stated: “Despite the lack of conclusive data,
it would seem reasonable that women whose GDM is
not well controlled, who require insulin, or who have
other risk factors such as hypertension or adverse obstetric history should be managed the same as individuals with preexisting diabetes. The particular antepartum test selected, whether nonstress test, contraction
stress test, or biophysical profile, may be chosen according to local practice”(34). In our institution GDM
mothers with risk factors noted above begin twice
weekly nonstress tests and amniotic fluid indices at between 32 and 36 weeks, depending upon the severity of
the risk factors. Those with no risk factors and whose
circulating glucose concentrations are within targets,
using medical nutrition therapy alone, start weekly
testing at 36 weeks.
FETAL GROWTH
The rate of macrosomia in GDM varies, depending
upon the diagnostic criteria and the method of treatment. In a randomized trial of identification and treatment of mild forms of GDM, macrosomia (birthweight
4000 g) was present in 21% (27) and 14% (28) of
untreated pregnancies, which was about twice the rate
in each study in pregnancies in which GDM was identified and treated. Because GDM is associated with fetal
macrosomia, and macrosomia in a fetus of a diabetic
mother is associated with an increased risk of shoulder
dystocia compared to the risk in a similar-weight fetus
of a nondiabetic mother, normalization of maternal
glucose is the most important means of prevention of
this problem. However, such efforts are not always successful, and large babies are sometimes born to mothers whose GDM is well controlled. Therefore periodic
ultrasound imaging of the fetus is used to estimate fetal
weight and growth trajectory. Caution should be exercised in interpreting ultrasound fetal weight estimations because the range of error is relatively wide. One
series of investigations has demonstrated the successful
use of ultrasound estimates of fetal growth trajectories
to determine which GDM mothers may or may not
benefit from insulin treatment with (57) or without
(58) increased fasting glucose concentrations.
TIMING OF DELIVERY
There is an increased risk of stillbirth in gestational
diabetic pregnancies, particularly when glucose concentrations are not within target ranges and the fetus
is presumably hyperinsulinemic. A 2011 workshop
jointly sponsored by the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the Society for Maternal-Fetal Medicine recommended that gestational diabetic pregnancies in
which glucose concentrations are well controlled, with
or without medication, not be delivered electively before 39 weeks (59). When GDM is poorly controlled
the timing of delivery is individualized and is generally
between 34 and 39 weeks, depending upon the situation. When all of almost 200 000 pregnancies complicated by GDM in California over a 10-year period were
analyzed, the stillbirth rate plus infant mortality rate
associated with delivery at various gestational ages was
compared to determine the risk of early delivery vs
waiting 1 more week(60). Such risks were not different
between 36 and 38 weeks, but at 39 weeks and beyond
the relative risk of expectant management exceeded
that of delivery. The absolute differences were small but
significant, with the number needed to deliver at 39
weeks (vs 40 weeks) to prevent a single excess death
being 1518. Because there is increased perinatal morbidity associated with early term delivery before 39
weeks (61), delivery between 39 and 40 weeks in cases
of gestational diabetic pregnancy appears to be a reasonable course. At our institution we recommend induction of labor for undelivered wo