What is the first clinical manifestation of magnesium toxicity in a pregnant woman?

The first clinical manifestation of magnesium toxicity in a pregnant woman is the loss of deep tendon reflexes. This occurs because magnesium is a neuromuscular blocker; it interferes with the transmission of signals at the neuromuscular junction. Elevated magnesium levels lead to a reduction in the excitability of both peripheral nerves and skeletal muscle, which can first be observed through the absence or diminished response of deep tendon reflexes, such as the patellar or Achilles reflex.

As magnesium toxicity progresses, other symptoms may emerge, including respiratory distress, altered mental status, and cardiac arrhythmias. However, these conditions typically develop later in the course of magnesium toxicity and are more severe manifestations. Thus, loss of deep tendon reflexes serves as an early warning sign, allowing for prompt intervention to prevent more serious complications associated with magnesium overload.