Normal variation in pregnancy: During the third trimester, the kidneys increase magnesium excretion to meet the growing fetus's needs. A 24-hour urine test showing 330mg of magnesium per day may be within normal limits for a pregnant woman, as pregnancy can affect renal handling of magnesium.

Dietary excess: High dietary intake of magnesium, possibly due to supplements or a diet rich in magnesium-containing foods, could lead to increased excretion.
Renal tubular dysfunction: Mild renal tubular dysfunction, which can occur in pregnancy, may impair the kidneys' ability to reabsorb magnesium, resulting in increased excretion.
Hyperparathyroidism: Although less common, hyperparathyroidism can increase magnesium excretion by increasing parathyroid hormone (PTH), which enhances renal magnesium excretion.

Primary hyperparathyroidism with severe hypercalcemia: Although rare, primary hyperparathyroidism can lead to significant hypercalcemia, which in turn increases magnesium excretion. Missing this diagnosis could have severe consequences for both the mother and the fetus.
Familial hypocalciuric hypercalcemia (FHH): A rare genetic disorder that affects calcium and magnesium metabolism, FHH could lead to increased magnesium excretion. Failing to diagnose FHH could result in unnecessary interventions or missing related complications.

Gitelman syndrome: A rare genetic disorder affecting the kidneys' ability to reabsorb magnesium and other ions, Gitelman syndrome could lead to elevated magnesium excretion. However, it is typically diagnosed earlier in life.
Bartter syndrome: Another rare genetic disorder affecting renal ion transport, Bartter syndrome might increase magnesium excretion but is usually identified in childhood.
Medullary sponge kidney: A rare congenital disorder affecting the kidneys, medullary sponge kidney could potentially lead to abnormalities in magnesium handling, although this is not a primary feature of the disease.