Use is recommended only if clearly needed and the benefit outweighs the risk.

AU TGA pregnancy category: D
US FDA pregnancy category: D

Comments:
-Intravenous use beyond 5 to 7 days can cause fetal abnormalities.
-Continuous IV infusion, especially more than 24 hours before delivery, can result in magnesium toxicity, including neuromuscular or respiratory depression, in the newborn.
-Use for preterm labor should be by obstetrical personnel in a hospital with appropriate obstetrical care facilities.
-Inform women that efficacy and safety for preterm labor has not been established, and that use beyond 5 to 7 days can cause fetal abnormalities.
-Avoid use in the 2 hours before delivery unless it is the only therapy available for eclamptic seizures.
-Monitor fetal heart rate.

There are no controlled data in human pregnancy. Magnesium sulfate readily crosses the placenta and may produce hypotonia and hypotension; fetal serum concentrations approximate those of the mother. Retrospective studies and case reports show fetal abnormalities such as hypocalcemia, skeletal demineralization, osteopenia, congenital rickets, and other skeletal abnormalities with continuous maternal administration for more than 5 to 7 days. Additionally, neonatal fractures have been reported. Increased pediatric mortality has been noted when used for pre-term labor. If administered in the 2 hours preceding delivery, the neonate may have signs of hypermagnesemia, including respiratory and neuromuscular depression.

AU TGA pregnancy category D: Drugs which have caused, are suspected to have caused or may be expected to cause, an increased incidence of human fetal malformations or irreversible damage. These drugs may also have adverse pharmacological effects. Accompanying texts should be consulted for further details.

US FDA pregnancy category D: There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.

See references

Safety has not been established; use only if clearly needed.

Excreted into human milk: Yes

Comments:
-The concentration of magnesium in breast milk is approximately twice that of maternal serum concentrations.
-50 to 90% of magnesium in breast milk is absorbed.
-Magnesium is cleared from breast milk within 24 hours of cessation of intravenous magnesium

See references