Osmolality (serum)
Serum osmolality decreases slightly in pregnancy due to physiologic reductions in sodium and increases in plasma volume.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mOsm/kg H₂O | 275 – 295 | 275 – 280 | 276 – 289 | 278 – 280 |
| mmol/kg | 275 – 295 | 275 – 280 | 276 – 289 | 278 – 280 |
Pregnancy physiology
- Plasma osmolality decreases by ~10 mOsm/kg due to hemodilution.
- Maternal sodium physiologically decreases by ~4–5 mEq/L.
- Lower osmolality stimulates ADH secretion reset, maintaining water balance.
- Changes are maximal by 10–12 weeks and persist throughout pregnancy.
Causes of low osmolality (hypo-osmolality)
- Normal pregnancy (physiologic)
- Hyponatremia (dilutional or true)
- SIADH (pain, stress, CNS disease)
- Heart failure, cirrhosis, nephrotic syndrome
- Excessive water intake / psychogenic polydipsia
- Adrenal insufficiency
- Oxytocin infusion (antidiuretic effect)
Causes of high osmolality (hyperosmolality)
- Dehydration (most common)
- Hypernatremia
- Hyperglycemia / diabetic ketoacidosis
- Mannitol therapy
- Alcohols (methanol, ethylene glycol) – ↑ osmolar gap
- Renal concentrating defects
Note: An elevated osmolar gap suggests toxic alcohol ingestion.
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.
- Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders. McGraw Hill.
- Guyton AC, Hall JE. Textbook of Medical Physiology.