Obstetrics and Gynaecology

at a Glance

Fourth EditionErrol R. Norwitz, John O. Schorge

Case Studies

Case 9: Gestational diabetes

A 28-year-old G4P2 presents to your office for a routine prenatal visit at 24 weeks’ gestation. Her physical examination is unremarkable and fetal wellbeing is reassuring. You recommend testing for gestational diabetes mellitus (GDM).

  • 1. What is GDM?

    Correct answer: GDM refers to any form of glucose intolerance with the onset of pregnancy or first recognized during pregnancy, and complicates approximately 5% of all pregnancies. It likely includes some women who have undiagnosed pregestational diabetes.

  • 2. Should everyone be screened for GDM? If so, at what gestational age should they be screened?

    Correct answer: Patients with GDM are typically asymptomatic. There is a small cohort of pregnant women in whom routine screening for GDM is not cost-effective. These are women under age 25 who have normal body mass index (BMI <25 kg/m2), no first-degree relatives with diabetes, no risk factors (such as a history of GDM, insulin resistance/PCOS [polycystic ovarian syndrome], a prior macrosomic infant, a prior unexplained late fetal demise, and women with persistent glycosuria), and who are not members of ethnic or racial groups with a high prevalence of diabetes (such as Hispanic, Native American, Asian, or African–American). As such patients are rare, most experts and organizations recommend screening for GDM in all pregnant women. The ideal time to screen for GDM is 24–28 weeks of gestation. For women at high risk of developing GDM (listed above), early screening for GDM is recommended at the first prenatal visit. If the early screen is negative, it should be repeated at 24–28 weeks.

  • 3. Her 1-hour GLT is 182 mg/dL. Does she have GDM?

    Correct answer: The most common screening test for GDM is the glucose load test (GLT) – also known as the glucose challenge test (GCT) – which is a non-fasting 50-g oral glucose challenge followed by a venous plasma glucose measurement at 1 hour. Most authorities consider the GLT to be positive if the 1-hour glucose measurement is >140 mg/dL. Use of a lower cut-off (such as >130 mg/dL) will increase the detection rate of women with GDM, but will result in a substantial increase in the false-positive rate.

    There is no GLT cut-off that should be regarded as diagnostic of GDM. A definitive diagnosis of GDM requires a 3-hour glucose tolerance test (GTT). In pregnancy, the GTT involves 3 days of carbohydrate loading followed by a 100-g oral glucose challenge after an overnight fast. Venous plasma glucose is measured fasting and at 1 hour, 2 hours, and 3 hours. Although there is agreement that two or more abnormal values are required to confirm the diagnosis, there is little consensus about the glucose values that define the upper range of normal in pregnancy (see below). Most institutions use the National Diabetes Data Group (NDDG) or Carpenter and Coustan cut-offs. Measurement of glycated hemoglobin (HbA1c) levels is not useful in making the diagnosis of GDM, although it may be useful in the diagnosis of pregestational diabetes.

    Diagnostic threshold for GDM during 100-g GTT


    Plasma glucose values (mg/dL) (mmol/L)*


    NDDG

    Sacks et al.

    Carpenter and Coustan

    Fasting

    105 (5.8)

    96 (5.3)

    95 (5.2)

    1-hour

    190 (10.6)

    172 (9.4)

    180 (9.9)

    2-hour

    165 (9.2)

    152 (8.3)

    155 (8.6)

    3-hour

    145 (8.1)

    131 (7.2)

    140 (7.7)

    * Values in parentheses are mmol/L.

  • 4. All four values of her 3-hour GTT are elevated and her fasting glucose level is 127 mg/dL. How would you manage her GDM? How long would you allow her to try dietary restriction before adding a hypoglycemic agent?

    Correct answer: GDM poses little risk to the mother. Such women are not at risk of diabetic ketoacidosis (DKA), which is primarily a disease of absolute insulin deficiency. However, GDM has been associated with an increase in infant birth trauma and perinatal morbidity and mortality. The risk to the fetus/infant is directly related to its size. Fetal macrosomia is defined as an estimated fetal weight (not birthweight) of ≥4,500 g. It is a single cut-off that is unrelated to gestational age, the sex of the baby, or the presence or absence of diabetes, or to the actual birthweight.

    The goal of antepartum treatment of GDM is to prevent fetal macrosomia and its resultant complications by maintaining maternal blood glucose at desirable levels throughout gestation, defined as a fasting glucose level <95 mg/dL and 1-hour postprandial level <140 mg/dL. Initial recommendations should include a diabetic diet (36 kcal/kg or 15 kcal/lb of ideal body weight plus 100 kcal per trimester given as 40–50% carbohydrate, 20% protein, and 30–40% fat to avoid protein catabolism), moderate exercise, daily home glucose monitoring, and weekly antepartum visits to monitor glycemic control. If diet alone does not maintain blood glucose at desirable levels, hypoglycemic therapy may be required. If initial fasting glucose levels are >95 mg/dL, treatment can be started immediately because “you can’t diet more than fasting.”

    Insulin (which has to be given several times a day by injection) remains the “gold standard” for the medical management of GDM. The use of oral hypoglycemic agents has traditionally been avoided in pregnancy because of concerns over fetal teratogenesis and prolonged neonatal hypoglycemia. However, recent studies suggest that second-generation hypoglycemic agents (glyburide, glipizide) do not cross the placenta, are safe in pregnancy, and can achieve adequate glycemic control in 85% of pregnancies complicated by GDM.

  • 5. The estimated fetal weight at 38 weeks’ gestation is 4,600 g (10 lb 2 oz). She has had six prior uncomplicated vaginal deliveries. How would you counsel her about delivery?

    Correct answer: As noted above, the complications of GDM are related primarily to fetal macrosomia, including an increased risk of cesarean section delivery, operative vaginal delivery, and birth injury to both the mother (vaginal, perineal, and rectal trauma) and fetus (including orthopedic and neurologic injury). Shoulder dystocia with resultant brachial plexus injury (Erb’s palsy) is a serious consequence of fetal macrosomia, and further increased in the setting of GDM because the macrosomia of diabetes is associated with increased diameters in the upper thorax of the fetus.

    The use of elective cesarean section delivery to reduce the risk of maternal and fetal birth injury in the setting of fetal macrosomia remains controversial. According to current ACOG guidelines, an elective cesarean section delivery at or after 39 weeks’ gestation should be recommended for all non-diabetic women who have a fetus with an estimated fetal weight (EFW) ≥5,000 g (or ≥4,500 g in a diabetic individual) to minimize the risk of birth trauma. Furthermore, it is recommended that a discussion be held about the safest route of delivery with non-diabetic women who have a fetus with an EFW ≥4,500 g (or ≥4,000 g in a diabetic individual) and that this discussion be documented in the medical record.

  • 6. After extensive counseling, the couple decline elective cesarean section delivery. She is now 38 weeks’ gestation. How should she be managed at this point in time?

    Correct answer: If the patient declines elective cesarean section delivery, spontaneous labor should be awaited. Induction of labor for so-called “impending macrosomia” does not decrease the risk of cesarean section delivery or intrapartum complications, and is therefore not routinely recommended. If she is still undelivered at 41 weeks’ gestation, she should be counseled again about induction of labor and/or elective cesarean section.

    During labor, maternal glucose levels should be maintained at 100–120 mg/dL to minimize the risk of intrapartum fetal hypoxic–ischemic injury. Continuous fetal monitoring is recommended throughout labor and the progress of labor should be carefully charted. Internal monitors such as an intrauterine pressure catheter (IUPC) and/or fetal scalp electrode can be used, if indicated. Neonatal blood glucose levels should be measured within 1 hour of birth and early feeding encouraged.

    Delivery of the fetus and placenta effectively removes the source of the anti-insulin (counter-regulatory) hormones that cause GDM. As such, no further management is required in the immediate postpartum period. A 2-hour non-pregnant GTT should be performed at 6–8 weeks postpartum in all women with GDM to exclude pre-gestational diabetes.

See Chapter 45.

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