Gestational diabetes occurs in 5% of pregnancies in the U.S., affecting more than 240,000 births annually.
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.
Most women should undergo a screening test for gestational diabetes 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.
To learn a little about treatment of gestational diabetes, check out this post.
Steve Parker, M.D.