Final answer:
The primary effect of maternal Mg2+ administration on a newborn is respiratory depression due to magnesium’s neuromuscular blocking properties. While magnesium can decrease maternal blood pressure, it does not improve uterine contractions, contrary to its use to prevent contractions in preterm labor. Increased fetal activity is unlikely as magnesium generally has a calming, not stimulating effect.
Step-by-step explanation:
When a mother is given Mg2+ during or before labor, the effects on a newborn can be significant. Although not explicitly stated in the resources, the evidence suggests magnesium plays a role in various bodily functions, which might impact the neonate. High levels of glucocorticoids, caused by magnesium deficiency, have been associated with altered insulin response and metabolic issues in newborn rats, according to studies by Takaya and others. Yet magnesium is typically used in obstetrics to relax the uterus and as an anticonvulsant, implying it could decrease uterine contractions, not improve them. The right option based on the medical use of magnesium sulfate in pregnancy is respiratory depression in the newborn, since magnesium can cross the placenta and affect the baby's neuromuscular function, potentially leading to decreased muscle tone and respiratory effort.
Magnesium sulfate is also used for its vasodilatory properties, which can decrease maternal blood pressure; however, this does not directly affect the newborn postpartum. Increased fetal activity is unlikely because magnesium has a calming effect. Therefore, the correct effect of maternal administration of Mg2+ on the newborn is respiratory depression, which reflects the neuromuscular blocking properties of magnesium when transferred to the baby.