What is a likely cause of hypomagnesemia in an asymptomatic twelve-hour-old neonate?

• A. Sepsis
• B. Placental insufficiency
• C. Maternal magnesium supplementation
• D. Delayed clamping of the umbilical cord

Answer: (B)

Magnesium moves from mother to fetus through the placenta, so a fetus relies on adequate placental blood flow to receive enough magnesium. If placental perfusion is insufficient, transfer of minerals, including magnesium, is reduced, making a newborn susceptible to hypomagnesemia after birth. In an asymptomatic twelve-hour-old infant, placental insufficiency explains the low magnesium as a result of limited transfer rather than a direct problem after birth.

Maternal magnesium supplementation would increase fetal magnesium levels, potentially causing neonatal hypermagnesemia with symptoms like hypotonia or respiratory depression, not hypomagnesemia. Delayed clamping raises blood volume and iron stores but does not specifically address magnesium transfer. Sepsis can affect many parameters but is not the most direct cause of isolated low magnesium in an otherwise well-appearing neonate.