Magnesium — Pregnancy Reference Values

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Magnesium (serum)

Serum magnesium may decrease slightly during pregnancy due to hemodilution, increased renal clearance, and fetal utilization.

Units Nonpregnant Adult 1st Trimester 2nd Trimester 3rd Trimester
mg/dL 1.5 – 2.3 1.6 – 2.2 1.5 – 2.2 1.1 – 2.2
mEq/L 1.25 – 1.9 1.33 – 1.83 1.25 – 1.83 0.92 – 1.83
mmol/L 0.63 – 0.95 0.67 – 0.92 0.63 – 0.92 0.46 – 0.92
Pregnancy physiology
  • Mild declines in serum magnesium occur due to increased GFR and renal excretion.
  • Hemodilution contributes to lower measured concentrations.
  • Fetal skeletal development increases maternal magnesium demands.
  • Magnesium remains within a relatively narrow physiologic range despite pregnancy changes.
Causes of elevated magnesium (hypermagnesemia)
  • Magnesium sulfate therapy (most common in pregnancy)
  • Renal insufficiency or acute renal failure
  • Adrenal insufficiency
  • Hypothyroidism
  • Excessive magnesium-containing antacids or laxatives
  • Tumor lysis syndrome / extensive cell injury

Symptoms usually appear when Mg > 4–5 mEq/L. Respiratory depression occurs at very high levels.

Cited references:
• Gronowski AM. Handbook of Clinical Laboratory Testing During Pregnancy. Humana Press, 2004. • Huijgen HJ et al. Magnesium in clinical laboratory medicine. Clin Chem Lab Med. • Cunningham FG, Williams Obstetrics (renal physiology chapter).

Causes of low magnesium (hypomagnesemia)
  • Increased renal losses (diuretics, renal tubular disorders)
  • Malnutrition, inadequate dietary intake
  • Malabsorption (celiac disease, Crohn disease, chronic diarrhea)
  • Alcohol use disorder
  • Uncontrolled diabetes / osmotic diuresis
  • Refeeding syndrome
  • Medications: PPIs, aminoglycosides, amphotericin B, cisplatin

Persistent hypomagnesemia increases risk of muscle cramps, arrhythmias, and hypocalcemia.

Cited references:
• Wallach J. Interpretation of Diagnostic Tests, 8th ed. • Swaminathan R. Magnesium metabolism and its disorders. Clin Biochem Rev. • Kayne LH et al. Maternal magnesium levels and fetal physiology. Am J Obstet Gynecol.

References

  1. Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.