Glycated Hemoglobin (Hemoglobin A1c - HbA1c)

The Hemoglobin A1c - HbA1c test is slowly becoming more frequently used in maternity care, but does it offer a better assessment of diabetes risk? In this article we'll cover the unique maternal physiology and how it may affect the accuracy of the HbA1c test. 

​HbA1c Testing

The glycated hemoglobin (hemoglobin A1c–HbA1c) test measures the percentage of proteins in the blood that are coated with glucose.

Glucose molecules in the bloodstream bind to hemoglobin, the protein in red blood cells responsible for carrying oxygen. This binding occurs randomly and spontaneously, without the need for enzymatic activity. Once glucose binds to hemoglobin, it forms a stable complex known as glycated hemoglobin or HbA1c. The extent of glycation depends on the concentration of glucose in the bloodstream and the lifespan of red blood cells. Since red blood cells have a lifespan of approximately 120 days, HbA1c reflects average blood glucose levels over the preceding two or three months.

​Due to physiological reasons around discrepancies in HbA1c values, it is currently not recommended as a substitute for OGTT.

​HbA1c in Pregnancy

Pregnancy leads to various physiological changes, including increased insulin resistance, hormonal fluctuations, and changes in blood volume. These changes can affect glucose metabolism and, consequently, HbA1c levels. In the first trimester, HbA1c levels may decrease due to increased blood volume and an expanded red blood cell mass. In the first trimester and the postpartum period, the discrepancy between average oral glucose tolerance test (OGTT) glucose and HbA1c-estimated average glucose is minimal. HbA1c decreases through the second trimester. As pregnancy progresses, insulin resistance increases, starting in the last weeks of the second trimester through the third trimester, and HbA1c levels rise. In the late second trimester, the average OGTT glucose is 13.2 mg/dL higher than , higher than the HbA1c-estimated average glucose. Lower hemoglobin concentrations explain this discrepancy.

After giving birth, HbA1c levels typically decrease. This can be due to the resolution of some of the physiological changes associated with pregnancy and the reduction of insulin resistance.

High ​HbA1c in Pregnancy

Elevated HbA1c is associated with maternal-fetal complications. 

 As of the writing of this book, there is not enough evidence to support the use of the HbA1c test as a replacement for the OGTT in diagnosing gestational diabetes. Evidence does indicate that higher HbA1c in the first trimester can be used as a predictive biomarker for increased risk of GDM later in pregnancy.

Most Common Causes of High HbA1c in Pregnancy

• High Cortisol
• Low Progesterone
• Thyroid Dysfunction
• Nutritional Deficiencies (vitamin D, vitamin B1 (thiamine), zinc)
• Iron Overload/Hemochromatosis
• Hypoxia
• Sleep Apnea/Snoring
• Chronic Infections/Inflammation
• Liver Disease/Inflammation

Studies have shown that Vitamin B12 deficiency causes falsely elevated HbA1c values.

Low HbA1c in Pregnancy

​There is limited data on the application of this indice in pregnancy.

Most Common Causes of Low ​HbA1c in Pregnancy

• Anemia
• Low Cortisol
• Nutritional Deficiencies (vitamin C, vitamin B6 (pyridoxine), vitamin B7 (biotin))
• Zinc Supplement Overdose
• Carrying Multiples
• Low Carbohydrate Diet
• Hypothyroidism

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Medical Disclaimer:
This content is for educational purposes only and is intended for licensed medical professionals. It is based on current research, subject to change, and not intended to diagnose, treat, or prevent any medical condition. This information does not replace consultation with a qualified healthcare provider. Seek professional medical evaluation and treatment from a licensed provider. Use of this information is at your own risk.​

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