Should I take magnesium in pregnancy?
Not routinely. The NHS asks for folic acid and vitamin D in pregnancy, not magnesium. Trials in 9,090 women found no clear benefit. Yet some women get magnesium by drip in hospital, for another reason.
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Should I take magnesium in pregnancy?
Most women don’t need to. The NHS asks you to take folic acid and vitamin D in pregnancy. Magnesium isn’t on that list.
For magnesium, the NHS says you should be able to get all you need from a varied and balanced diet. Spinach, nuts and wholemeal bread all have it.
Women aged 19 to 64 need 270mg a day. The NHS gives no separate amount for pregnancy.
| Supplement | NHS advice in pregnancy |
|---|---|
| Folic acid | 400 micrograms a day, from trying for a baby to 12 weeks. Some need a higher GP dose. |
| Vitamin D | 10 micrograms a day, early October to late March. Some may need it all year. |
| Iron | Only if your GP or midwife recommends it. |
| Vitamin A (retinol), cod liver oil | Don’t take. Check the label. |
| Magnesium | Not on the list. A varied diet should cover it. |
Sources: NHS: pregnancy supplements; NHS: magnesium
Think you need something else? The NHS says to speak to a GP about other supplements.
Start with folic acid and vitamin D. See folic acid for women.
Is a magnesium supplement safe in pregnancy?
There’s no NHS dose limit just for pregnancy. For adults in general, the NHS says 400mg or less a day from supplements is unlikely to cause any harm.
More than 400mg for a short time can cause diarrhoea. And there isn’t enough evidence to say what high doses do over a long time.
- 400 mg or belowNHS: unlikely to cause any harm (adult advice).
- Above 400 mgNHS: can cause diarrhoea.
Source: NHS: magnesium
Add up the magnesium in every supplement you take, including any multivitamin. Then check the same labels for vitamin A or retinol, which the NHS says can harm your baby’s development.
Thinking of starting one? Ask your midwife or GP first. See can you take too much magnesium and which magnesium to take.
Does magnesium help in pregnancy?
Not clearly. A Cochrane review pooled 10 trials in 9,090 women who took magnesium tablets or none.
| What trials looked at | What they found |
|---|---|
| Pre-eclampsia (high blood pressure in pregnancy) | No clear difference. 3 trials, 1,042 women. |
| Baby smaller than expected | No clear difference. 3 trials, 1,291 babies. |
| Leg cramps | Mixed results in small single trials. |
Sources: Cochrane 2014; Cochrane 2020
Only 2 of the 10 trials were judged high quality. The verdict: there’s not enough high-quality evidence that magnesium supplements in pregnancy help.
For cramps, some single trials found magnesium may help and others found it may make little or no difference. The review says it’s unclear whether any treatment works, and it couldn’t judge their safety.
NICE tells doctors not to recommend magnesium just to prevent high blood pressure in pregnancy. Ask your midwife before trying magnesium for cramps. More in magnesium for cramps.
Why do hospitals give magnesium in pregnancy?
That’s magnesium sulfate, a medicine given by drip into a vein. It does a different job from a tablet, in two hospital situations.
| Situation | What NICE says |
|---|---|
| Severe pre-eclampsia or very high blood pressure in critical care (intensive care), with a fit (seizure) now or before | Give a magnesium sulfate drip. |
| Severe pre-eclampsia in critical care, birth planned within 24 hours | Consider a magnesium sulfate drip. |
| 24 weeks to 29 weeks and 6 days pregnant, and in labour or with birth planned within 24 hours | Offer a drip to help protect the baby’s brain. |
| Trying to prevent high blood pressure in pregnancy | Don’t recommend magnesium supplements for this. |
Sources: NICE NG133; NICE NG25
The NHS says women with severe pre-eclampsia might be given medicine to prevent seizures as an injection or drip.
A tablet at home can’t copy this. What helps instead is knowing the pre-eclampsia warning signs, listed at the end of this guide.
How can I ease cramp or constipation in pregnancy?
These are often why people reach for magnesium. The NHS suggests trying these first.
NHS tips for cramp and constipation in pregnancy
- Bend and stretch each foot up and down 30 times.
- Rotate each foot 8 times one way and 8 times the other.
- During a cramp, pull your toes hard up towards your ankle, or rub the muscle hard.
- Eat high-fibre foods: wholemeal bread and cereals, fruit, vegetables, beans and lentils.
- Drink plenty of water and exercise regularly.
- On iron tablets and constipated? Ask your doctor about managing without them or a different type.
Gentle ankle and leg movements may help prevent cramp, which is most common at night.
Never stop or change a prescribed medicine without your prescriber. The NHS says check with your doctor first, as stopping could harm you and your baby.
When to speak to a GP
Your midwife is also a good first call in pregnancy. Get help if:
- You have signs of pre-eclampsia: a severe headache that simple painkillers don’t ease, blurred vision or flashing lights, pain below the ribs, sudden swelling of the face, hands or feet, feeling very unwell, heartburn that heartburn medicines don’t help, or being sick. Get urgent help: call your maternity unit, or 111 if you can’t.
- Leg cramps are disturbing your sleep, last longer than 10 minutes, or come with numbness or swelling in your legs. See a GP.
- You’re on iron tablets and constipated. Ask your doctor about a different type.
- You think you need a supplement other than folic acid and vitamin D. Speak to a GP.
Worried about low magnesium? See signs of low magnesium. For other pregnancy supplements, see omega-3 in pregnancy.