Magnesium During Pregnancy: Which Type Is Safest and How Much to Take
Updated 11 April 2026
Pregnant women need more magnesium than at any other time in their lives, yet most do not get enough. Glycinate is the preferred form: it is gentle on an already sensitive stomach, supports sleep during pregnancy, and helps prevent leg cramps. Here is everything you need to know about safe supplementation by trimester.
Always consult your OB or midwife
This guide provides general information about magnesium during pregnancy. Every pregnancy is different. Before starting any new supplement, confirm with your prenatal healthcare provider that it is appropriate for your specific situation, especially if you have gestational diabetes, preeclampsia risk factors, or are taking other medications.
Why Magnesium Matters in Pregnancy
The recommended daily allowance for magnesium increases to 350 to 360 mg during pregnancy (up from 310 to 320 mg for non-pregnant women). This increased demand serves several critical functions.
Fetal bone and tissue development
Magnesium is essential for bone mineralisation and protein synthesis in the developing fetus. It works alongside calcium and phosphorus to build the skeletal system. The fetus accumulates approximately 750 to 1,000 mg of magnesium during gestation, drawn from maternal stores.
Preeclampsia risk reduction
Intravenous magnesium sulfate is the standard medical treatment for eclampsia and severe preeclampsia. While oral magnesium supplementation is not a treatment for these conditions, some evidence suggests that adequate magnesium intake during pregnancy may help reduce the risk of developing preeclampsia. A Cochrane review by Makrides et al. found that magnesium supplementation during pregnancy was associated with lower rates of preeclampsia, preterm birth, and low birth weight, though the authors noted the evidence quality was moderate.
Leg cramp prevention
Leg cramps affect up to 50% of pregnant women, particularly in the second and third trimesters. The increased blood volume, growing uterus compressing nerves, and higher magnesium demand all contribute. Magnesium supplementation has shown mixed but generally positive results for reducing pregnancy-related cramps.
Sleep support
Sleep disruption is nearly universal in pregnancy, especially in the third trimester. Physical discomfort, anxiety, and hormonal changes all contribute. Magnesium glycinate supports sleep through both the magnesium component (GABA enhancement, cortisol reduction) and the glycine component (core temperature regulation, brainstem inhibition). This makes it particularly valuable during pregnancy.
Glycinate vs Citrate During Pregnancy
Glycinate: preferred
- + Gentle on the stomach (critical during pregnancy)
- + Supports sleep (glycine benefit)
- + Reduces anxiety and stress
- + No laxative effect
- + Well-tolerated alongside prenatal vitamins
Citrate: acceptable
- + Helps with pregnancy constipation
- + More affordable
- ! Watch for diarrhoea and dehydration
- ! Start at a very low dose
- ! Take in the morning, not before bed
Constipation is extremely common during pregnancy (affecting 40 to 50% of women) due to progesterone slowing gut motility and iron supplements. If constipation is your primary issue, citrate can help. But start with a very low dose (100 mg elemental) because dehydration from diarrhoea is a more serious risk during pregnancy than it is normally.
Dosage by Trimester
First trimester (weeks 1 to 12)
Nausea is common, so glycinate's gentle GI profile is especially important. Start with 100 mg of elemental magnesium glycinate per day and increase gradually. Take with a small meal to minimise any chance of nausea. If you are already taking a prenatal vitamin, check whether it contains magnesium (many contain 50 to 100 mg) and adjust your supplement dose accordingly.
Second trimester (weeks 13 to 26)
Nausea typically subsides. Leg cramps often begin. Increase to 150 to 200 mg of elemental magnesium from supplements (alongside dietary intake of approximately 200 mg from food). This is when cramp prevention and sleep support become most valuable. Split the dose: half with lunch, half 1 to 2 hours before bed.
Third trimester (weeks 27 to 40)
Sleep disruption peaks. Leg cramps may worsen. Magnesium demand is at its highest. Continue 150 to 200 mg of elemental magnesium from supplements. The evening dose of glycinate is especially helpful for sleep in the third trimester. If constipation becomes a major issue, you may switch the morning dose to citrate and keep glycinate at night.
Magnesium and Morning Sickness
There is limited but suggestive evidence that magnesium may help reduce nausea in early pregnancy. Magnesium plays a role in smooth muscle relaxation (including the stomach) and neuromuscular function. Some practitioners recommend it as part of a nausea management protocol alongside B6, ginger, and small frequent meals.
If you are taking magnesium for morning sickness, glycinate is the only sensible choice. Citrate's laxative effect would make nausea worse, not better. Oxide is also poorly tolerated. Take a low dose (100 mg elemental glycinate) with a small snack. If it helps, continue. If it does not help within a week, it is unlikely to be the solution for your nausea.
Forms to Avoid During Pregnancy
Magnesium oxide
Poor absorption (4 to 5% bioavailability). More likely to cause bloating, gas, and GI discomfort. Not worth the GI risk during pregnancy when better-absorbed forms are available.
High-dose citrate
While low-dose citrate is fine, high doses (above 300 mg elemental) can cause significant diarrhoea and dehydration, which are particularly dangerous during pregnancy. Never use medical bowel prep citrate during pregnancy.
Unregulated or untested supplements
During pregnancy, choose supplements from reputable brands with third-party testing (USP, NSF, ConsumerLab). Avoid supplements with proprietary blends that do not clearly list magnesium content.
Disclaimer: This content is for educational purposes only and is not a substitute for prenatal medical care. Always consult your OB-GYN, midwife, or healthcare provider before starting any supplement during pregnancy.