Research regarding the positive and negative effects of supplementation of magnesium during pregnancy are mixed.
In general, it is believed pregnant women consume low amounts of magnesium through food, which may not be enough to reach the recommended amount of the nutrient during pregnancy. However, evidence is not strong enough to recommend routine supplementation in women who are not deficient.
Supplementation may be recommended for women with Crohn’s disease, celiac disease, or diabetes, in which their body may have trouble absorbing magnesium.
Pregnant women should not take magnesium supplements without talking to their health care provider (HCP), as it is very easy to take too much, and can result in nausea, vomiting, diarrhea, and/or cramping.
Note: Evidence regarding the use of supplemental magnesium for the prevention/treatment of nausea and vomiting of pregnancy is lacking. Additionally, if women are not deficient in magnesium, taking any additional magnesium could make nausea and other gastrointestinal symptoms worse.
Women should talk to their HCP if they have any questions.
Background
Magnesium is the fourth most abundant mineral in the body and plays a role in the functioning of the muscles and organs. It also impacts electrolyte balance, protein synthesis, nerve function, blood sugar levels, and blood pressure regulation.
Calcium is needed for muscle tension; magnesium is needed for relaxation. If the calcium concentration in the muscle increases due to a lack of magnesium, the muscle cannot relax, and cramps occur. Magnesium helps cramps by making cell membranes resistant to calcium, potassium, and sodium, which helps restore electrolyte balance.
Pregnancy
Magnesium supplementation during pregnancy has been studied since the 1980s and remains controversial, with positive and negative associations. Supplementation during pregnancy has been advocated due to the lack of magnesium in the diets of pregnant women in general, with the hope of treating some pregnancy-related complications.
Although there are suggested links between magnesium deficiency and gestational diabetes, preterm labor, high blood pressure, leg cramps, preeclampsia, edema, and small for gestational age or intrauterine growth restriction, others argue there is not enough evidence to recommend magnesium to prevent those complications.
However, studying supplementation is difficult because researchers do not agree on magnesium requirements during pregnancy, or how magnesium is used during pregnancy.
Some researchers believe magnesium levels decline during pregnancy, and that pregnancy is a state of magnesium depletion; others believe the opposite, and that the fetus contains about 1 gram of magnesium at term, with most of its gains in the second and third trimesters.
Deficiency (General)
Low magnesium levels in the body can occur from: a diet constantly low in magnesium, chronic alcoholism, Crohn’s disease, celiac disease, people with insulin resistance and/or type 2 diabetes, high doses of zinc, and/or long-term treatment with loop diuretics or proton pump inhibitors that do not contain magnesium (for heartburn/reflux).
Early signs of magnesium deficiency include muscle and leg cramps, loss of appetite, nausea, vomiting, fatigue, weakness, and unbalanced calcium and vitamin D levels. These signs/symptoms could potentially worsen during pregnancy.
Severe magnesium deficiency can lead to numbness, tingling, muscle contractions and cramps, seizures, personality changes, abnormal heart rhythms, and coronary spasms.
Based on the above, magnesium supplements are sometimes marketed as cure-alls simply based on what deficiency appears to cause. However, evidence to support these claims is scarce when a person is not deficient (i.e. “extra” magnesium likely has no effect).
Supplementation
Some studies assessing magnesium supplementation during pregnancy have indicated positive effects with doses ranging from 128 milligrams (mg) to 730 mg. Various international organizations recommend a magnesium dietary allowance for pregnant women between 240 to 480 mg. However, the tolerable upper limit in the United States for magnesium supplementation has been set at 350 mg, showing a wide disagreement and interpretation of evidence related to magnesium during pregnancy.
Magnesium from prenatal supplements is rarely more than 50 to 100 mg, if magnesium is included at all.
Phillips’ Milk of Magnesia® provides 500 mg magnesium per tablespoon, with dosage instructions advocating 4 tablespoons per day (some of the magnesium is not absorbed because of the laxative effect, but still remains above the upper limit).
Extra-strength Rolaids® provides 55 mg magnesium per tablet (Tums® does not contain magnesium).
Symptoms of toxicity include hypotension, nausea, vomiting, facial flushing, retention of urine, intestinal blockage, depression, and lethargy before progressing to muscle weakness, difficulty breathing, extreme hypotension, irregular heartbeat, and cardiac arrest.
Magnesium supplements are available in a variety of forms, including magnesium oxide, lactate, aspartate, citrate, and chloride. There are no reported safety studies of different doses of magnesium supplements during pregnancy.
Unlike supplements, too much magnesium from food does not cause overdose symptoms because excess amounts are eliminated in urine; further, only approximately 30% to 40% of the magnesium consumed through food is absorbed by the body.
Food Sources
Food surveys show that individuals in the United States likely do not eat enough foods rich in magnesium to obtain recommended amounts, despite magnesium being present in a variety of foods such as: green leafy vegetables (spinach), legumes, nuts, seeds, whole grains, fortified cereals, almonds, edamame, black beans, pistachios, avocado, potatoes, brown rice, and yogurt.
Action
Women who are concerned about their magnesium status or overall diet during pregnancy should speak with their HCP. Pregnant women should never take any supplements without speaking to their HCP first.
If women are experiencing constipation, a common symptom of pregnancy, they should ask their HCP about laxatives/stool softeners, some of which do contain magnesium. When used appropriately, certain laxatives can be used safely during pregnancy. Read more here.
Resources
Magnesium Fact Sheet (U.S. National Institutes of Health)
Magnesium (Oregon State University, Micronutrient Information Center)