Is Too Much Magnesium Bad for a Fetus?

Your body uses magnesium to feed your muscles and organs, and it also helps to regulate energy and mineral levels. You get magnesium from foods like leafy greens, whole grains and nuts, but eating an unbalanced diet still can leave you with low levels. Compound this with the fact that your magnesium need rises when you're pregnant, and the fact that many pregnant people stray from straight-and-narrow nutritionally, and you have a clear cause for a supplement. Some obstetricians include magnesium supplements as part of prenatal care to help alleviate preeclampsia and leg cramps, but it's important to supplement only with your doctor's supervision.

Women typically need 310 to 350 milligrams of magnesium per day, but the physical burden of growing a fetus increases those needs. Pregnant people 19 to 30 years old need 350 milligrams per day, pregnant people over 31 need 360 milligrams, and pregnant people 14 to 18 years old need 400 milligrams. The risk of consuming too much from food alone is very small, but taking a supplement without first speaking to your doctor raises your risk.

Magnesium supplement come in many forms and they are not interchangeable. Magnesium sulfate, or Epsom salt, is considered very safe during all trimesters, although newborns might show symptoms of toxicity if the mother was given magnesium sulfate before the birth. Magnesium carbonate is used only when the benefit outweighs the risk because it has not been studied with regard to fetal health. Magnesium citrate is prescribed only when absolutely necessary because quality studies in pregnant people don't exist and there always is a potential for harm. In some cases though, the benefit is greater than the risk, in which case your doctor will provide explicit instructions for use.

The National Institutes of Health Office of Dietary Supplements recommends capping magnesium supplement doses at 350 milligrams per day for pregnant people. This is considered the largest amount that can be safely tolerated, and the assumption is that any difference between the cap and your daily needs will be met through food. A lower dose would be better and would be sufficient if you follow a balanced diet during your pregnancy -- a single ounce of dry roasted almonds provides 80 milligrams of magnesium, and 3 ounces of halibut provides 90, so it's not difficult to meet your needs with minimal supplementation.

The effects of magnesium sulfate supplementation on newborns are not considered serious -- the most major effects are treated with three to five days of intravenous calcium. The biggest concern is calcium depletion, because magnesium interferes with calcium absorption. As a result, there is a small risk of rickets and chronically low calcium levels in the fetus, and an even smaller risk of respiratory distress. That said, a magnesium overdose severely affects the mother and can harm the fetus indirectly. Simple side effects like nausea and diarrhea usually won't hurt the baby, but a severe intoxication can produce respiratory paralysis, coma, cardiac arrest and death. Healthy babies come from healthy mommies, so consult your doctor before using a magnesium supplement.