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Magnesium in pregnancy: needs, benefits and limits

Clinical review pending

How much magnesium pregnancy requires, what studies show on leg cramps and complications, and what remains open about supplements, tests and safety.

The body needs more magnesium during pregnancy. The US reference values used here put the total at 350 to 400 mg a day from food and other sources combined, depending on age. That does not mean an extra magnesium supplement is necessary. Whether such supplements relieve leg cramps or prevent pregnancy complications remains unclear. Intravenous magnesium sulfate given in hospital is a different matter: it is used in severe preeclampsia to protect against seizures, and before an expected very early birth to protect the baby’s brain.

Why magnesium needs rise during pregnancy

At the recommended total daily intake, magnesium is considered safe during pregnancy as well. Under the US reference values, the recommended amount rises from 310 to 320 mg for women who are not pregnant to 350 to 360 mg during pregnancy; under the age of 19 it is 400 mg. As a so-called cofactor, magnesium helps with more than 300 enzymatic reactions, including making DNA and proteins and energy metabolism. These processes run at a higher pace in pregnancy. The growing baby continuously needs magnesium from the mother’s body to build bones, muscles and nerve tissue. NIH reference values

Prenatal multivitamins with 50 to 100 mg of magnesium cover only part of this total. How much more is needed also depends on how much magnesium the diet provides.

The mother’s blood volume increases by 40 to 50%, which dilutes the magnesium in her blood. At the same time, the kidneys filter more fluid per unit of time. This so-called glomerular filtration rate rises, and more magnesium is lost in the urine. The higher reference value accounts for both the changes in the mother’s body and the baby’s growth.

If intake is not enough, the body can release magnesium from the mother’s bones and soft tissues to supply the baby. Stores can decline without this being noticeable at first. Muscle cramps or irritability are described as possible symptoms. A pooled analysis of studies in pregnant African women found lower magnesium levels in the serum, the liquid part of the blood, in women with preeclampsia. This does not tell us whether low magnesium status contributes to the development of this pregnancy complication. Systematic review and meta-analysis

Magnesium is rarely part of routine lab tests in pregnancy. When it is measured, it is usually in serum. Less than 1% of the body’s total magnesium is in the blood. A normal serum value therefore does not rule out depleted tissue stores.

What clinical studies show about magnesium and pregnancy complications

Oral magnesium supplements and intravenous magnesium sulfate serve different purposes. In preeclampsia with severe features, intravenous magnesium sulfate is part of standard treatment. It cuts the risk of eclamptic seizures by roughly half. One explanation offered for this protection is magnesium’s action as a calcium antagonist: it stabilizes the membranes of nerve cells and reduces vascular spasms in the brain. The US Food and Drug Administration (FDA), however, advises against using magnesium sulfate for extended periods to relax the uterus in preterm labor.

Shortly before an expected preterm birth before 32 weeks of pregnancy, magnesium sulfate is given to protect the baby’s brain. Several randomized controlled trials showed that this lowers the risk of cerebral palsy, a permanent disorder of movement and posture, in preterm babies who survive. Review on fetal neuroprotection

Oral magnesium has also been studied for preventing preeclampsia and preterm labor. The studies included pregnant women at different levels of risk:

  • A review of 10 studies found not enough high-quality evidence that magnesium supplements prevent preeclampsia or restricted fetal growth. A later meta-analysis of seven studies, by contrast, found a moderate reduction in the risk of preeclampsia, mainly in studies of women at higher risk. Review of 10 studies, meta-analysis of seven studies
  • One study using 365 mg of magnesium citrate daily during the first 18 weeks of pregnancy is reported to have found less preeclampsia. This finding was not consistently confirmed across all studies. The exact length of supplementation and what the amount refers to remain unclear from this description of the study.
  • For leg cramps in pregnancy, the results are inconsistent and the certainty of the evidence is low. Whether oral magnesium relieves the symptoms therefore remains unclear. Cochrane review on leg cramps

One possible link to cramps is magnesium’s role in signaling between nerves and muscles and in muscle relaxation. With a deficiency, nerve cells become easier to excite and muscle contractions can increase. That could encourage cramps, but it does not yet explain whether taking extra magnesium during pregnancy helps.

How magnesium might act on cramps, preeclampsia and preterm labor

How magnesium affects the excitability of nerves and muscles

Magnesium affects N-methyl-D-aspartate receptors, NMDA receptors for short. They are involved in excitatory signaling between nerve cells. With magnesium deficiency, these receptors can become overactive. Nerve cells then fire more often, and muscles are more easily excited. One possible result is spontaneous muscle contractions, in other words cramps.

Magnesium also regulates the sodium-potassium pump, which maintains the electrical differences between the inside and outside of muscle cells. With too little magnesium it works less efficiently, which can make involuntary contractions even more likely. Whether extra magnesium relieves pregnancy cramps cannot be concluded from these mechanisms alone, and the studies mentioned have not yet answered the question clearly either.

Magnesium as a calcium antagonist in preeclampsia

In preeclampsia, the inner lining of the blood vessels does not work normally, the vessels narrow and blood pressure rises. Magnesium acts as a calcium antagonist: it blocks calcium channels in the smooth muscle of the vessel walls and can thereby counteract excessive narrowing. In the brain, reduced vascular spasms and the calming of overexcitable nerve cells are cited as explanations for the protection against seizures.

Other possible effects under discussion include stabilizing the cells of the vessel lining and reducing oxidative stress and inflammation. These mechanisms, however, do not establish that supplements reliably prevent preeclampsia.

Relaxing the uterus and possible relevance in preterm labor

The muscle layer of the uterus, the myometrium, contracts when calcium flows into its cells. Magnesium inhibits this influx and can thereby reduce the frequency and strength of contractions. This mechanism explains why magnesium sulfate has been used as a labor-suppressing drug, also called a tocolytic. Its effectiveness is limited, however, and it is no longer recommended for longer-term suppression of labor.

Another assumption is that adequate magnesium status helps keep the uterine muscle relaxed during pregnancy. On this view, a deficiency could encourage premature contractions. Whether magnesium supplements can prevent preterm births has not been settled by this.

Dose, form and timing: what is known so far

Form

Magnesium compounds differ in how well the body absorbs them. Magnesium oxide is common in over-the-counter supplements and has low bioavailability. It holds water in the gut, which means it acts osmotically, and it can cause diarrhea. Glycinate and citrate are considered better absorbed. Whether they are also better tolerated, especially in pregnancy, cannot be judged with confidence here.

In magnesium glycinate, magnesium is bound to the amino acid glycine. A bond like this is called a chelate. The idea that glycinate could support sleep and muscle relaxation rests partly on the calming properties attributed to glycine. That does not establish any such benefit in pregnancy. Citrate is also well absorbed and has a mild laxative effect, which can help with pregnancy-related constipation. That does not, however, show that it is the safest form for pregnant women.

Dose

The recommended total daily intake depends on age. The figures below are US reference values. The upper limit for supplements is a separate thing: it does not describe the total daily requirement. NIH: Magnesium

Age during pregnancy Magnesium per day
14–18 years 400 mg
19–30 years 350 mg
31–50 years 360 mg
Upper limit from supplements 350 mg

If a prenatal multivitamin contains 50 to 100 mg of magnesium, it supplies part of the recommended total. Food, and additional supplements where appropriate, can contribute the rest. The multivitamin’s content alone does not show that there is a shortfall.

Nuts, seeds, legumes, whole grains and green leafy vegetables supply magnesium.

Studies on leg cramps used 300 to about 365 mg of magnesium a day, split into two or three doses. One study on preventing preeclampsia is reported to have used 365 mg of magnesium citrate a day; exactly what this amount refers to is still unclear. The upper limit of 350 mg applies to extra sources outside ordinary food. Magnesium from food does not count toward it. Study amounts may exceed this limit and are not an individual dosing recommendation. Review on leg cramps

Timing

One argument for taking magnesium with a meal is that stomach acid makes magnesium more soluble and so improves its absorption. No general rule for all supplements can be derived from this, however.

Because leg cramps mostly happen at night, taking magnesium in the evening seems a natural choice. Possible relaxing and sleep-promoting effects are also cited in support. The studies mentioned do not show whether magnesium actually works better in the evening during pregnancy than at another time of day.

The advice to take magnesium at a different time from calcium or iron supplements is based on possible competition for absorption. Without details of the product and dose, it remains open whether a gap is needed.

Combinations

Magnesium is needed to activate vitamin D in the liver and kidneys. In turn, vitamin D is credited with promoting magnesium absorption in the gut. These connections explain why the two are considered together. They do not answer whether extra magnesium improves the benefit of a vitamin D supplement.

Vitamin B6 is also said to play a role in how the body uses magnesium. It has also been studied in connection with pregnancy-related nausea and leg cramps. That does not yet add up to a recommendation for a combination product.

Who might benefit from magnesium supplements, and when caution is needed

Whether a supplement might make sense depends on factors such as diet, possible absorption problems and medications. Extra magnesium is discussed for the following situations, but a benefit has not been shown for every group:

  • A suspected magnesium deficiency based on a red blood cell measurement. How reliably this test detects a deficiency remains an open question, however.
  • Leg cramps or muscle spasms.
  • An increased risk of preeclampsia or preterm labor.
  • Inadequate dietary intake, or gastrointestinal conditions that impair absorption, such as celiac disease or inflammatory bowel disease.
  • Taking medications that can lower magnesium status, including proton pump inhibitors and certain diuretics.
  • Preeclampsia in a previous pregnancy, chronic high blood pressure or kidney disease. With kidney disease in particular, though, caution is needed because magnesium can build up.
  • A multiple pregnancy, where a higher need is assumed because more than one baby is developing.

For leg cramps and for preventing preeclampsia or preterm labor in particular, the benefit of taking extra magnesium remains uncertain. A risk factor alone is therefore not a reason to recommend supplements.

When kidney function is impaired, magnesium can reach toxic levels, because the body excretes it mainly through the kidneys. Possible consequences include muscle weakness, low blood pressure and heart rhythm disturbances. In myasthenia gravis, high-dose magnesium should also be avoided, because it can worsen muscle weakness.

During intravenous treatment with magnesium sulfate, for example for preeclampsia or to suppress labor, the medical team closely monitors reflexes, breathing rate and urine output. This allows early signs of toxicity to be spotted.

Measuring magnesium status: can you track the effect of a supplement?

Magnesium is most often measured in serum. The value gives only a limited picture of the whole body’s supply, though: less than 1% of the body’s magnesium is in the blood. The serum value can therefore be normal even when the stores inside cells are depleted.

An RBC magnesium test measures the magnesium content of red blood cells (erythrocytes). The idea is that this value might better reflect magnesium stores in tissue. Whether it is actually more informative than serum magnesium remains open.

The following symptoms are described with magnesium deficiency in pregnancy:

  • Muscle cramps, especially in the calves.
  • Irritability and mood swings.
  • Fatigue and sleep problems.
  • With more pronounced deficiency, heart rhythm disturbances.

Repeated RBC measurements before and during supplementation can capture changes in the magnesium content of red blood cells. Whether this reliably shows that intake is adequate, or that supplementation has a health benefit, has not been established.

Vitamin D, calcium and parathyroid hormone are also looked at for additional context, because they are linked with magnesium in metabolism. For inflammatory markers such as high-sensitivity C-reactive protein, hs-CRP, the underlying assumption is that chronic inflammation increases magnesium use and could deplete stores. Whether such additional values improve the assessment of magnesium supply remains open.

RBC magnesium is usually not part of routine lab tests in pregnancy. That alone does not suggest a deficiency is being missed, and it is no reason for regular RBC checks.

Frequently asked questions

Why can magnesium deficiency occur in pregnancy?

The need increases because the growing baby requires magnesium for bones, muscles and nerve tissue. At the same time, the mother’s body changes: blood volume increases by 40–50%, which dilutes the magnesium in the blood. The kidneys filter more fluid per unit of time, the glomerular filtration rate rises, and more magnesium is excreted.

If dietary intake is not enough, the body can release magnesium from the mother’s bones and soft tissues to supply the baby. Stores can decline unnoticed at first, before symptoms appear. These connections say nothing, however, about how common deficiency is in pregnancy.

What signs and symptoms can occur with low magnesium in pregnancy?

Nighttime leg cramps, especially in the calves, are described, as well as irritability, mood swings, fatigue and poor sleep. With more severe deficiency, heart rhythm disturbances can occur.

Low magnesium levels have also been observed in preeclampsia; this does not prove a causal link. Because less than 1% of the body’s magnesium is in the blood, serum levels can be normal even when tissue stores are depleted.

How does magnesium sulfate help prevent seizures in preeclampsia?

In women with preeclampsia with severe features, intravenous magnesium sulfate lowers the risk of eclamptic seizures by roughly 50%. The clinical regimen described is a loading dose of 4–6 g followed by an infusion of 1–2 g per hour. This treatment requires close medical monitoring; these figures are not instructions for self-administration.

Magnesium acts as a natural calcium antagonist. It stabilizes the membranes of nerve cells and reduces vascular spasms in the brain that can contribute to seizures.

Which form of magnesium is safest in pregnancy and best absorbed?

The comparisons described do not identify a single safest form. Glycinate and citrate are considered better absorbed than magnesium oxide, which is common in over-the-counter supplements. Better absorption, though, does not answer the question of safety in pregnancy.

In glycinate, magnesium is bound to the amino acid glycine. Because glycine is credited with calming properties, possible support for sleep is also discussed. Whether pregnant women benefit from this remains open. Citrate is also well absorbed and has a mild laxative effect, which can help with pregnancy-related constipation. The assumption that both forms cause fewer gastrointestinal side effects than oxide is not supported here specifically for pregnant women.

What is RBC magnesium, and is the test more informative than a standard serum test?

RBC magnesium is the magnesium content in red blood cells. The test is meant to reflect tissue stores better than serum magnesium. Whether it is actually more informative remains unresolved, however.

Less than 1% of the body’s total magnesium is in the blood. The serum value can therefore be in the normal range even when stores in cells and tissues are significantly depleted. RBC measurements before and during supplementation capture whether the magnesium content of red blood cells changes. It does not automatically follow that they also reliably show adequate intake or a benefit from supplementation.

How much magnesium is needed each day in pregnancy?

The US reference values give a total daily intake of 350–360 mg during pregnancy, compared with 310–320 mg for women who are not pregnant. For pregnant women under 19, the value is 400 mg. If a prenatal multivitamin contains 50–100 mg of magnesium, it supplies only part of this amount. Food, and additional supplements where appropriate, can cover the rest. Whether there is a shortfall therefore also depends on diet. NIH reference values

The rough difference of 250–300 mg between the reference value and the content of such a supplement does not fit every combination of the figures given. Above all, it ignores magnesium from food and so does not describe any need for additional supplementation.

Studies on leg cramps used 300 to about 365 mg of magnesium a day, split into two or three doses. These study amounts are not a personal dosing recommendation. They may exceed the stated US upper limit of 350 mg from extra sources. Cochrane review

Can magnesium supplements stop leg cramps in pregnancy?

So far, that cannot be said reliably. Studies of oral magnesium have produced inconsistent results, and the certainty of the evidence is low.

One possible mechanism involves signaling between nerves and muscles. Magnesium deficiency increases the excitability of nerve cells and impairs the sodium-potassium pump, which can make involuntary muscle contractions more likely. This link alone does not show that a supplement relieves pregnancy cramps.

The evening is considered as a time to take it because the symptoms mostly occur at night. That does not establish an advantage over other times of day.

What role does magnesium play in preterm labor?

The uterine muscle, the myometrium, contracts when calcium flows into its cells. Magnesium inhibits this influx and can thereby reduce the frequency and strength of contractions. This is the basis for the assumption that adequate magnesium status helps keep the uterus relaxed during pregnancy.

Magnesium sulfate was therefore used as a labor-suppressing drug. Its effectiveness is limited, however, and longer-term use is not recommended. Giving it before an expected preterm birth before 32 weeks of pregnancy serves a different purpose: it is meant to lower the risk of cerebral palsy in babies who survive. This protection of the baby’s brain does not mean that the preterm birth itself is prevented.

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