Injections of magnesium sulfate were used to stop pre-term labor.
The FDA [1,2] is now warning about administering it for more than 5 days, because of a risk to damage the bone development of the foetus. Even more troubling is that the FDA has not determined that injecting Magnesium sulfate for less than 5 days will not result in bone structure abnormalities and hypocalcemia.
Hypocalcemia is a condition where the calcium levels in the blood serum are too low.
Another use of magnesium sulfate in OB-GYN is injection against risk of seizures in preeclampsia and eclampsia
Those are life-threatening complications that can occur during pregnancy. Preeclampsia can lead to eclampsia, seizures, stroke, multiple organ failure, and death of the woman and/or baby.
Early pregnancy intervention for eclampsia or advancing pre-eclampsia include abortion.
But, if the pregnancy is sufficiently advanced to allow for delivery of the baby, labor induction or Caesarean section would be used [3.] Keep in mind that that a premature baby has a 50% chance to survive at 24 weeks [4.]
Magnesium sulfate is a treatment to prevent seizures in women with pre/eclampsia , and therefore gave more time for the pregnancy to progress. Because, by giving more time before delivery, the fetus’s lungs can mature , although corticosteroids seem more effective 
Other methods include medication to lower the blood pressure (anti-hypertensive) and aspirin [3.]
In pre-eclampsia, there are conflicting needs between bringing the baby to a reasonably viable delivery time (34 weeks, according to ), and increasing risk to the mother.
The administration of magnesium sulfate, protected the mother against seizures, and gave the baby’s lungs more time to mature. Conversely the pre-eclampsia has more time to progress, and monitoring it was key for the decision to keep the pregnancy going, or not.
The Doctors might have lost one important tool to bring the baby to a reasonable delivery time, since the 5 days (FDA safe limit for baby bones) of administration of magnesium sulfate , does not seem like much time at all.
Talk to your Doctor
Medscape ; “Extended Use of Magnesium Sulfate Can Harm Fetus, FDA Says” ; May 30 2013
FDA ; “Magnesium Sulfate: Drug Safety Communication – Recommendation Against Prolonged Use in Pre-term Labor” ; May 30 2013
Pediatrics ; “Medical Staff Guidelines for Periviability Pregnancy Counseling and Medical Treatment of Extremely Premature Infants” ; Jan 2006
Healthline ; “Management of Preeclampsia During Delivery” ; March 2012
Healthline ; “Treatment of Preeclampsia: Magnesium Sulfate Therapy” ; March 2012
Gynecologia y obstetricia de Mexico ; “The induction of fetal lung maturity in pre-eclampsia” ; 1999
Medscape ; “Pre-eclampsia, Severe Pre-eclampsia and Hemolysis, Elevated Liver Enzymes and Low Platelets Syndrome”; 2011