Normal physiologic changes of pregnancy are the most likely cause of this pregnant patient's decrease in her serum sodium level. While the plasma volume increases during pregnancy, water retention exceeds the concomitant sodium retention, resulting in mild hypo-osmolality and hyponatremia. The plasma osmolality typically decreases by 8 to 10 mOsm/kg H₂O, and the serum sodium decreases by 4.0 mEq/L (4.0 mmol/L). These hormonally mediated changes in plasma osmolality and serum sodium do not require therapy and resolve following delivery.

Cerebral salt wasting can be mistaken for the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and can occur in response to central nervous system disease (particularly subarachnoid hemorrhage) and closed head injury. It is associated with an increased loss of sodium by the kidney, resulting in hyponatremia. However, these changes occur over a longer timeframe than seen in this patient with recent head trauma.

Pituitary apoplexy is acute hemorrhage into the pituitary gland and would be a consideration in this patient. If severe, it may cause headache, vision changes due to ocular nerve compression, and hypopituitarism. All pituitary hormonal deficiencies can occur, including adrenocorticotropic hormone (ACTH) deficiency, which may lead to cortisol deficiency and hypotension if it occurs rapidly. Although the resulting cortisol deficiency may result in mild hyponatremia, this would not occur immediately after injury. Additionally, this patient has a normal blood pressure for pregnancy and no other symptoms or clinical findings suggesting this diagnosis.

Neurologic conditions such as a closed head injury from a motor vehicle accident can cause the SIADH, but hyponatremia would not develop acutely upon initial presentation as in this patient.