When monitoring a patient on magnesium sulfate for preterm labor, a developing sign of toxicity would be?

When administering magnesium sulfate for preterm labor, monitoring for signs of toxicity is critical to ensure both maternal and fetal safety. A significant indicator of magnesium toxicity is the absence of deep tendon reflexes. This phenomenon occurs because magnesium is a central nervous system depressant, and high levels can diminish neuromuscular function.

The absence of deep tendon reflexes suggests that the magnesium level in the patient's bloodstream may be excessively high, impacting nerve transmission and muscle function. This finding necessitates immediate intervention, which may include stopping the magnesium infusion and initiating appropriate treatment to reverse the effects of magnesium toxicity.

In contrast, the other options do not indicate magnesium toxicity effectively: an increased respiratory rate could occur due to a variety of other factors, including anxiety or a respiratory condition; a normal urinary output of 30 mL/hour is generally acceptable unless much lower, indicating potential complications; and worsening abdominal pain could arise from numerous causes unrelated to magnesium levels specifically. Therefore, the absence of deep tendon reflexes is the most reliable sign of potential magnesium sulfate toxicity, providing a clear actionable outcome for the healthcare provider.