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Sodium (Na⁺)

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Trimester-Specific Reference Ranges

Pregnancy slightly lowers serum sodium due to plasma volume expansion and a reduced osmotic threshold for vasopressin release.

Units Nonpregnant Adult 1st Trimester 2nd Trimester 3rd Trimester
mEq/L 136–146 133–148 129–148 130–148
mmol/L 136–146 133–148 129–148 130–148
Physiologic Changes in Pregnancy
  • Lower osmotic threshold for vasopressin → mild physiologic hyponatremia
  • Plasma volume expansion dilutes serum sodium
  • Typical sodium range in pregnancy: 134–138 mEq/L
  • Placental vasopressinase increases ADH clearance
Causes of Low Sodium (Hyponatremia)
  • SIADH
  • Preeclampsia or eclampsia
  • Hyperemesis gravidarum
  • Adrenal insufficiency
  • Thiazide diuretics
  • Excess free water intake
Causes of High Sodium (Hypernatremia)
  • Dehydration
  • Gestational diabetes insipidus (vasopressinase-mediated)
  • Osmotic diuresis
  • High sodium intake or hypertonic fluids
References
  1. Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies. Obstet Gynecol. 2009;114:1326–1331.
  2. Davison JM. Renal hemodynamics and fluid balance in pregnancy. Clin Obstet Gynecol. 1980.
  3. Lindheimer MD, Barron WM. Osmoregulation in pregnancy. Am J Physiol. 1989.
  4. Fischbach FT, Dunning MB. Manual of Laboratory and Diagnostic Tests. LWW.