Review: Gestational Diabetes

You can’t tell if she has it just by looking

Ever heard of gestational diabetes?  It’s when a when a woman develops diabetes during pregnancy.  It usually goes away soon after the baby is born.  All pregnancies are characterized by some degree of insulin resistance and high insulin levels: they are necessary for the baby.  Nevertheless, healthy pregnant women run blood sugars 20% lower than when they are not pregnant.

In the U.S., gestational diabetes occurs in 5% of pregnancies, affecting more than 240,000 births annually.  Compared to caucasians, gestational diabetes mellitus (GDM) occurs more often in blacks, native Americans, Asians, and Latinos.

So What’s the Big Deal?

Numerous problems are associated with GDM, for both the mother and the baby:

• dangerously high blood pressure (preeclampsia)
• excessive amount of amniotic fluid (the baby in the uterus floats in this fluid)
• delivery requiring an operation
• early or premature delivery
• death of the baby
• birth trauma, such as broken bones or nerve injury
• metabolic problems in the baby (low blood sugar, for example)
• abnormally large baby (macrosomia, a major problem)

Diabetic ketoacidosis—a life-threatening complication of diabetes—is rare in GDM.

How Is GDM Diagnosed?  (section updated December 28, 2013)

Most women should undergo a screening test around the 24th to 28th week of pregnancy.  Screen earlier if undiagnosed type 2 diabetes is suspected or if risk factors for diabetes are present.  The American Diabetes Association (2014 guidelines) recommends either one of two screening tests.

• “One-step test.” It’s a morning oral glucose tolerance test after at least eight hours of fasting. Fasting blood sugar is tested then he woman drinks 75 grams oral of glucose.  Blood sugar is tested again one and two hours later.  This blood sample is obtained by a needle in a vein, not by finger prick.  Gestational diabetes is diagnosed if any of the following apply: 1) fasting glucose is 92 mg/dl (5.1 mmol/l) or higher, 2) 0ne-hour level is 180 mg/dl (10.0 mmol/l) or higher, or 3) two-hour level is 153 mg/dl (8.5 mmol/l) or higher.
• “Two-step test.” This is a nonfasting test with only one needle-stick. The woman drinks 50 grams of glucose; plasma glucose is tested one hour later. But if it’s over 140 mg/dl (10.0 mmol/l), that’s a flunk and a three-hour 100-gram oral glucose tolerance test in the fasting state must be done (step two). Gestational diabetes is present if the three-hour glucose is 140 mg/dl (7.8 mmol/l) or higher. Other experts say the diagnosis requires two or more of the following:

• fasting blood sugar > 95 mg/dl (5.3 mmol/l)
• 1-hour blood sugar > 180 mg/dl (10 mmol/l)
• 2-hour blood sugar > 155 mg/dl (8.6 mmol/l)
• 3-hour blood sugar > 140 mg/dl (7.8 mmol/l)

You’ll find that various expert panels have proposed different criteria for the diagnosis. The National Institutes of Health in the U.S. published their consensus statement in 2013. 

There’s no need for the screening test if a random blood sugar is over 200 mg/dl (11.1 mmol/l) or a fasting sugar is over 126 mg/dl: those numbers already define diabetes, assuming they are confirmed with a second high reading.  A random blood sugar over 200 mg/dl (11.1 mmol/l) should probably be repeated for confirmation.  Gestational diabetes can be diagnosed at the first prenatal visit if fasting blood sugar is 92 or over mg/dl (5.1 mmol/l or over) but under 126 mg/dl (7 mmol/l), or if hemoglobin A1c at the first prenatal visit is 6.5% or greater.

Women with diabetes in the first trimester have overt diabetes, not gestational diabetes.

What’s the Treatment for Gestational Diabetes?

Briefly: diet, exercise, blood glucose self-monitoring,  and insulin if needed.  The immediate goal is to achieve normal blood sugars.

A registered dietitian is involved in teaching diet modification.  The standard recommendation is to reduce carbohydrate consumption to 35-40% of total calories.  [By comparison, the standard American diet provides about 55% of calories as carbohydrates.]  Protein and fat are about 20% and 40%, respectively.  Total calorie recommendations are based on the mother’s ideal body weight.  Insulin resistance is greatest in the morning, so breakfast is small.  Complex carbohydrates are favored over simple sugars.  Most women will achieve normal glucose (blood sugar) levels with diet modification.

Moderate exercise also helps control blood sugars, partially by increasing sensitivity of body tissues to insulin.

How often should blood sugar be monitored by the mother?  Ideally, at least four times daily: fasting (on an empty stomach before breakfast) and one hour after the first bite of each meal.

Insulin therapy should be seriously considered when fasting glucose is over 90–95 mg/dl (5.0–5.3 mmol/l) or 1-hour-after-meal glucose is over 120-130 mg/dl (6.7–7.2 mmol/l).  Fifteen percent of mothers with GDM will need insulin injections.  If elevated fasting glucose is the only problem, NPH insulin before bedtime is prescribed.  If sugar levels one hour after meals are elevated, then use rapid-acting insulin aspart or insulin lispro just before meals.  In some cases, both NPH and rapid-acting insulin are both necessary.

Outside the U.S., glucose-lowering pills are sometimes used: glyburide and/or metformin most commonly.

What About After Delivery?

Having had gestational diabetes, the mother is at high risk of developing typical non-pregnant diabetes in the future. She should be screened for the development of diabetes and prediabetes at least every three years. Regular exercise and loss of excess weight decrease the risk substantially.

Steve Parker, M.D.

Sources:  UpToDate.com article on gestational diabetes, accessed March, 2010 and October, 2011.  American College of Obstetrics and Gynecology position paper published in 2001.  Diabetes Care, vol. 34, 2011: supplement 1: S62.  Blood sugar levels on the 3-hour 100-gram glucose tolerance test are from the Fourth International Workshop-Conference on Gestational Diabetes, published in 2000. For gestational diabetes diagnosis: American Diabetes Association’s Standards of Care in Diabetes – 2014.