Glycated hemoglobin in pregnancies at increased risk for gestational diabetes mellitus

Objective

The glycated hemoglobin (HbA_1c) value is increasingly used for the detection of (pre)diabetes, but HbA_1c decreases during pregnancy. We sought to identify clinical and metabolic correlates of HbA_1c in pregnancies at increased risk for gestational diabetes mellitus (GDM).

Study design

We prospectively studied 335 gravidas who received a 3-h 100 g oral glucose tolerance test (OGTT) at 24–32 weeks, in most cases after an abnormal glucose challenge test. Several indices of insulin sensitivity and secretion were computed from fasting measurements and the OGTT.

Results

HbA_1c concentrations gradually increased in diet-treated and insulin-treated GDM gravidas compared with non-GDM gravidas. HbA_1c was higher if the insulin peak was delayed until 180 min compared with 60 or 120 min. Stepwise regression identified the homeostasis modeling assessment of insulin resistance (HOMA-IR) as the first-rank correlate. Other correlates were ethnicity, a low insulin-to-glucose response at 60 min, and gestational age. The HbA_1c value corresponding to a fasting glucose of 5.1 mmol/l (diagnostic of GDM) was 2 mmol/mol (∼0.2%) higher if sampling occurred at 29–32 vs. 24–28 weeks or if ancestry was non-European vs. European.

Conclusion

HbA_1c is strongly associated with insulin resistance; in addition, HbA_1c captures the first-phase insulin response. However, HbA_1c varies with gestational age and ethnicity.