Fasting Glucose

The serum glucose test is found on both the comprehensive metabolic panel and basic metabolic pane. It is generally recommended that these panels be run fasting for accuracy. In this article we will discuss how maternal physiology affects the fasting glucose test, as well as the most common causes of abnormal ranges. 

Fasting Glucose Testing

Serum glucose measures the amount of glucose in the bloodstream. Levels are influenced by the amount of carbohydrates and sugar in the diet, stress, inflammation, and hormones, and regulated by insulin. 

Fasting Glucose in Pregnancy

Glucose levels undergo several changes during pregnancy, primarily due to the various hormonal and metabolic adaptations that occur to support the growing fetus and the changing needs of the mother’s body.

During the first trimester, cells are quickly dividing to create the fetus and placenta. This requires quick and instant energy in the form of glucose. During early pregnancy, human chorionic gonadotrophin (hCG) promotes insulin sensitivity. This means that maternal cells become more responsive to insulin, and the body utilizes glucose more effectively. This increased sensitivity to insulin is thought to help facilitate nutrient transfer from the mother to the developing fetus. Blood glucose levels decrease. Compared to preconception levels, fasting glucose levels decrease by a median of 3 mg/dL in the first trimester. This returns to normal preconception levels in the second trimester. During the third trimester, there is another slight decrease in serum glucose levels due to an increase in insulin production.

Optimal obstetrical outcomes are seen in those with a fasting blood glucose of 90 mg/dL or less. 

As pregnancy advances, the placenta begins to produce placental lactogen. Lactogen affects insulin sensitivity in the maternal tissues, creating a state of insulin resistance. This state of insulin resistance can be seen in the second trimester, and as pregnancy progresses, the sensitivity of insulin receptors decreases by 50%. In addition, throughout the course of pregnancy, to compensate for the increased demand for sugar in fetal growth and maturity, insulin production can be up to fifteenfold what it was preconception, with an average of 3- to 3.5-fold increase in fasting insulin levels.

Insulinase is an enzyme that plays a role in glucose metabolism during pregnancy. It is produced in the placenta and has the ability to degrade insulin. Insulinase production begins to increase as the placenta matures and grows during the second trimester. By the third trimester, insulinase production is at its peak. Insulin degradation is highest during this period, contributing to significant insulin resistance in pregnant women. This insulin resistance helps to ensure that glucose is readily available to the fetus, even as maternal insulin levels rise.

After birth, fasting glucose levels typically decrease gradually over the first few days and weeks of postpartum. This decline is partly due to the cessation of placental lactogen and insulinase. As these hormones diminish, insulin sensitivity improves, leading to lower fasting glucose levels. 

Because of the drastic change in fasting blood glucose levels in early pregnancy, first-trimester screening of fasting glucose via the comprehensive metabolic panel offers the opportunity to detect and treat undiagnosed pregestational diabetes.

​This becomes a major problem, as the prevalence of diabetes increases rapidly. Otherwise, these high-risk women would not receive any special treatment until the beginning of the third trimester. If blood-sugar dysregulation issues are addressed early, overt GDM may be prevented via dietary, lifestyle, and supplemental management.

High Fasting Glucose in Pregnancy

First-trimester fasting blood glucose levels may be a predictive marker of gestational diabetes and poor pregnancy outcomes. A fasting blood glucose level of 4.6 mmol (millimole)/L (approximately 82 mg/dL) was associated with an increased risk of gestational diabetes mellitus (GDM) later in pregnancy.

Other studies have found a cutoff around 84 and 85 mg/dL in the first trimester as a predictive marker of GDM later in pregnancy.
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The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study found strong associations between higher levels of maternal glucose between 24 and 32 weeks, within what is currently considered normoglycemia, and adverse pregnancy outcomes. Lower ranges of fasting blood sugar, <90 mg/dL, is considered protective for both maternal and fetal health in pregnancy but also with childhood metabolic health, as those with higher fasting glucose levels showed long-term effects on childhood metabolic function.

Most Common Causes of High Fasting Glucose in Pregnancy

• Preexisting diabetes
• Preexisting Insulin Resistance
• Iron Overload/Hemochromatosis
• T3 Syndrome 
• Hyperthyroidism
• Acute Physical Trauma
• Infection/Inflammation
• High Cortisol
• Hypoxia
• Sleep Apnea/Snoring
• Liver Disease
• Pancreatitis
• Vitamin C Overdose
• Nutritional Deficiencies (vitamin A, vitamin D, vitamin B1 (thiamine), magnesium, zinc)

Low Fasting Glucose in Pregnancy

If the fasting glucose is <75 mg/dL and there are symptoms of hypoglycemia, such as nausea and vomiting in pregnancy, fasting hypoglycemia should always be suspected.

​If fasting glucose is <70 mg/dL, fasting hypoglycemia is definite.

Most Common Causes of Low Fasting Glucose in Pregnancy

• Carrying Multiples (higher hCG)
• Secondary Hypothyroidism via Hypopituitarism
  
• Excess Intense Exercise (Overexercising)
  
• History of Bariatric Surgery
  
• Certain Medications (Beta-Blockers, Antibiotics, Metformin) 
• Acute Infections
• Nutritional Deficiencies (vitamin B7 (biotin), vitamin B6 (pyridoxine), vitamin C)
• Zinc Supplementation Overdose

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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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