Magnesium oil is safe to use during pregnancy and often essential. Its often used IV to stop premature pregnancy too. Its helpful to prevent pre-eclampsia it also has anti-seizure properties and anti-hypertensive properties.
Only caution is if you have kidney dysfunction, then only use it under your doctor supervision.
Magnesium oil has not been tested by FDA standards because it is NOT a pharmaceutical product, it is a natural product. Few natural products are so tested.