Here's link to the answer:
tinyurl.com/wpazsebu
If there is suspected hypomagnesemia present that’s the time
that you can use the magnesium. This is for use in cardiac arrest. This kind of drug is life threatening
ventricular arrhythmias due to digitalis toxicity. There is insufficient
evidence to recommend for or against the routine administration of magnesium
during cardiac arrest. Magnesium is for the treatment of documented
hypomagnesemia or for torsade de pointes.
If I am correct it is The Third Trimester