Magnesium

Pregnancy

The National Academies' 1997 Dietary Reference Intakes report sets pregnancy RDAs of 400 mg/day for ages 14 to 18, 350 mg/day for ages 19 to 30, and 360 mg/day for ages 31 to 50. ODS reproduces these values and explains that RDAs count magnesium from food, beverages, supplements, and medicines. The adult UL is 350 mg/day from supplements and medicines only; food magnesium is excluded. EFSA's 2015 opinion uses a different reference system, an AI of 300 mg/day for adult women, and applies the same AI to pregnancy and lactation. The EFSA AI is not a conflicting U.S. RDA. Evidence that extra oral magnesium prevents pregnancy complications is limited. The 2014 Cochrane review included 10 randomized trials with 9,090 women and babies. Across trials, it found no significant difference in perinatal mortality, small for gestational age, or preeclampsia; only two trials were judged high quality. Some secondary outcomes favored magnesium, but results were mixed and some came from one large trial. The review concluded there was not enough high-quality evidence to show benefit. The 2020 BRAMAG double-masked randomized trial compared 300 mg/day oral magnesium citrate from 12 to 20 weeks until delivery in higher-risk singleton pregnancies. Of 911 randomized participants, 407 in the magnesium group and 422 controls were analyzed. The perinatal composite occurred in 18.4% versus 18.0% (adjusted OR 1.10, 95% CI 0.72 to 1.68); the maternal composite occurred in 12.0% versus 9.7% (adjusted OR 1.29, 95% CI 0.83 to 2.00). Recruitment stopped short during a Zika outbreak; post hoc power for the planned effect was 34.9%. The investigators concluded that supplementation did not appear to reduce adverse perinatal or maternal outcomes. CNPq and the Gates Foundation funded the study and reported no role in its design, data, analysis, publication, or preparation. The 2026 LIFE Child prospective cohort included 1,078 pregnancies and found no association between reported magnesium supplementation and pregnancy duration. It was observational and did not assess dietary magnesium intake or biochemical deficiency. A separate 2026 observational study of 233 women evaluated magnesium lactate with pyridoxine for low serum magnesium; treated and untreated findings were limited and inconsistent. A 2026 meta-analysis of circulating magnesium and pregnancy loss did not study supplementation, found no statistically significant pooled difference, and rated certainty very low. Its authors cautioned that it should not be interpreted as evidence for or against routine supplementation. Keep clinical IV treatment separate from nutrition. FDA's 30 May 2013 communication advises against magnesium sulfate injection for more than 5 to 7 days to stop preterm labor. It reports low fetal or newborn calcium and bone findings, including osteopenia and fractures, and says the shortest duration that can cause harm is unknown. MHRA's 2019 update also describes possible neonatal skeletal and biochemical effects after prolonged or repeated maternal use and advises considering neonatal monitoring. Holcomb et al. studied 11 neonates after more than 7 days of IV tocolysis; six exposed neonates had radiographic long-bone findings, compared with none of 22 unexposed controls. ACOG states IV magnesium sulfate should be used for seizure prevention and treatment in severe-feature gestational hypertension or preeclampsia, or eclampsia. A 2026 Cochrane review of IV magnesium sulfate for fetal neuroprotection is another distinct clinical use, not evidence about dietary or routine oral supplements.

Breastfeeding

LactMed's magnesium oxide entry was revised 15 February 2024. It says no information is available on clinical magnesium oxide use during breastfeeding, while concluding that it can be taken without special precautions. LactMed says oral absorption of magnesium by an infant is poor, but it also reports no published infant magnesium-level data. The infant bowel study by Baldwin (1963) compared 50 mothers who received senna with 50 controls who received mineral oil or mineral oil with magnesia. The paper does not state how many controls received magnesia. All infants received supplemental feeds except one. The study assessed bowel patterns, not infant serum magnesium, and its mixed control group does not establish a magnesium-specific infant effect. Meier et al. (2005) compared 40 matched pairs of healthy women with vaginal singleton births. The magnesium group took oral magnesium aspartate hydrochloride, 15 to 30 mmol/day, for at least four weeks until delivery; the reported mean was 459 mg/day, with a range of 365 to 729 mg/day. Fewer mothers in the magnesium group exclusively breastfed at discharge (24 versus 34, P=0.04). This was a small matched observational study, not a randomized trial, and the studied doses exceeded the adult supplemental UL. It does not establish an infant blood-level effect or a result for every magnesium salt. Solly et al. (2026) reviewed 11 studies with 2,842 participants who received postpartum IV magnesium sulfate for maternal hypertension. Extended administration was associated with delayed breastfeeding initiation, delayed maternal perception of secretory activation, and greater reliance on milk expression. The review is about hospital IV treatment and not routine oral supplements. LactMed separately summarizes that postpartum IV magnesium sulfate can delay the onset of lactation compared with shorter courses. ODS says high doses from supplements or medicines can cause diarrhea, nausea, and abdominal cramping; the 350 mg/day adult UL applies to supplemental and medicinal sources, not food. Direct evidence for routine oral magnesium use while breastfeeding remains sparse.

What was tested

Pregnancy research includes randomized trials and a Cochrane review of oral magnesium supplements, the BRAMAG trial of magnesium citrate, and recent observational studies. These studies do not establish a clear benefit from extra supplementation to prevent pregnancy complications. Breastfeeding research includes a mixed-product infant bowel study, a small matched study of oral magnesium aspartate through delivery, and an integrative review of postpartum IV magnesium sulfate. The IV studies concern clinical treatment and should not be applied to routine oral supplements.