Quick answer

NIH lists an upper limit of 350 mg a day for magnesium from supplements and medicines during pregnancy, the same as for other adults. A Cochrane review of 10 trials found too little high-quality evidence to show a benefit and flagged one result to treat with caution. Ask your doctor before taking a supplement.^1,2

Key facts

NIH's upper limit for supplemental magnesium is 350 mg a day for pregnant women aged 14 to 50. It counts supplements and medicines, not food.¹
NIH lists recommended amounts from all sources in pregnancy of 400 mg at ages 14 to 18, 350 mg at 19 to 30 and 360 mg at 31 to 50.¹
The Cochrane review pooled 10 trials with 9090 women. Only two were judged high quality, and the authors found too little high-quality evidence to show a benefit.²
LactMed says magnesium supplements taken in pregnancy might delay the onset of lactation.³
Over-the-counter magnesium citrate and magnesium oxide labels tell pregnant users to ask a health professional or doctor first.^4,5

What NIH says about magnesium amounts in pregnancy

NIH's fact sheet for health professionals gives a recommended amount of magnesium for pregnancy that depends on age: 400 mg a day for pregnant teens and 350 to 360 mg for pregnant adults. These amounts count magnesium from food, drinks, supplements and medicines together.¹

The upper limit works differently. It covers only magnesium from supplements and medicines, and NIH lists it at 350 mg a day for pregnant women from ages 14 to 50. Magnesium that occurs naturally in food is not counted against it.^1,6

The upper limit for magnesium from supplements and medicines is 350 mg a day for adults. It does not count magnesium in food and drinks.^1,6

NIH notes that supplements can be useful when needs are hard to meet from food, for example during life stages such as pregnancy.⁶

What a Cochrane review found about magnesium supplements in pregnancy

A Cochrane review pooled 10 randomized or quasi-randomized trials of oral magnesium in pregnancy. The forms, doses and starting weeks differed from trial to trial, and the review included oxide, citrate, gluconate, aspartate and stearate. Only two trials were judged high quality.²

Main results of the Cochrane review of magnesium supplements in pregnancy²
Outcome	Result
Stillbirth and newborn death before discharge	No significant difference (risk ratio 1.10, range 0.72 to 1.67)
Small for gestational age	No significant difference (risk ratio 0.76, range 0.54 to 1.07)
Pre-eclampsia	No significant difference (risk ratio 0.87, range 0.58 to 1.32)
Newborn death before discharge, a secondary result	Higher with magnesium (risk ratio 2.21, range 1.02 to 4.75), mostly from one trial

For that last row, the review's authors said one trial supplied over 70% of the infants and that severe birth defects and deaths of twins in that trial likely explained it. When the deaths from birth defects were removed, no higher risk of newborn death was seen. The authors still advised caution in reading the result, and concluded there was not enough high-quality evidence to show that magnesium supplements in pregnancy are beneficial.²

What labels and LactMed add

The Drug Facts label of an over-the-counter magnesium citrate laxative says that anyone who is pregnant or breastfeeding should ask a health professional before use. The label of a magnesium oxide antacid tablet puts pregnancy and breastfeeding on its list of reasons to ask a doctor before use.^4,5

NIH's LactMed database, which is written for breastfeeding, also notes that magnesium supplements taken during pregnancy might delay the onset of lactation. The entries describe a matched study of magnesium aspartate in late pregnancy, summarized on the breastfeeding page.^3,7

Who should ask their doctor before using magnesium in pregnancy

NIH's own advice for any supplement in pregnancy is caution beyond a standard prenatal supplement, because many have not been well tested in pregnant women. Ask your doctor or midwife if any of these apply.⁸

You have kidney disease. NIH says the risk of magnesium toxicity rises when the kidneys cannot clear the excess.¹
You take antibiotics such as tetracyclines or quinolones, an oral bisphosphonate, a diuretic or a medicine for acid reflux. NIH names these as medicines magnesium may interact with or affect.¹
Your prenatal vitamin or an antacid you use already contains magnesium. Magnesium is in multivitamin and mineral products as well as in some laxatives and heartburn remedies.^1,6
You have leg cramps and are considering magnesium for them. The Cochrane review of magnesium for cramps found conflicting results in pregnancy.⁹

Questions people ask

Is magnesium safe during pregnancy for sleep?

The sources on this page do not address sleep in pregnancy. The Cochrane review measured outcomes for mothers and babies, and a four-week sleep trial of magnesium bisglycinate did not include pregnant or breastfeeding women. Ask your doctor before using a supplement for any reason.^2,10

Is magnesium ok during pregnancy?

NIH sets an upper limit for supplemental magnesium in pregnancy and lists recommended amounts, and a Cochrane review found too little high-quality evidence to show a benefit. Your doctor can say whether it fits your situation.^1,2

Is magnesium safe in pregnancy, according to the NHS?

This page uses US sources: NIH, DailyMed labels, LactMed and a Cochrane review. It does not report what the NHS says, so check that page or ask your midwife or doctor.¹

Is magnesium safe in early pregnancy or the first trimester?

The Cochrane review's summary does not break its results down by trimester, and the trials began magnesium at different points. The sources checked do not give a separate answer for the first trimester, so ask your doctor.²

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.