Why you need it, and how much
Magnesium is important for creating and renewing bone, protein and fatty acids, it also relaxes muscles - all of which are important during pregnancy. Magnesium helps your body build and repair all the body tissue it needs as you develop and support your growing baby. Meanwhile, the relaxing properties of magnesium for muscles is particularly important for the uterus, where it helps to prevent the premature contracting of the womb by relaxing the uterine muscles, so decreasing the risk of premature labour. Here magnesium works in tandem with calcium: whereas calcium stimulates muscles to contract, magnesium helps them to relax.
The most serious possible consequences of a magnesium deficiency during pregnancy are an increased risk of both pre-eclampsia and premature labour. At a more general level, getting enough magnesium will make you feel healthier and help you cope with general pregnancy discomforts. Apart from being vital to hundreds of bodily functions, magnesium can help reduce the cramping that is so common in pregnancy, decrease the intensity of Braxton Hicks contractions and also help with constipation.
The 'right amount' of magnesium during pregnancy is around 350mg daily.
Diets high in fat and sugar and low in whole grains, vegetables and fruits are likely to be low on magnesium.
Where to get it
As with other vitamins and minerals, the best way to get your magnesium during pregnancy is through a healthy, balanced diet. Magnesium is found naturally in leafy vegetables, dates, apricots, nuts, seeds, avocado, bran, wholewheat breads and dairy products.
Some breakfast cereals are fortified with magnesium and different mineral waters will have different levels of magnesium: some mineral waters are particularly rich in magnesium, so if you drink a lot of bottled water it may be worth comparing your brand with others.
You can check magnesium content on the nutrition information of packaging, but with fresh foods the usual rule of 'the fresher, the better' holds true. Magnesium is also lost through cooking, so minimal cooking or eating raw sources of magnesium is advisable.
If for any reason you're worried that you're not getting enough magnesium, then you can take a supplement. Specific antenatal vitamin tablets will include magnesium, but perhaps in only very low doses, so it may make more sense to take a stand-alone tablet.
If you do take a magnesium supplement then be aware that it can inhibit the absorption of iron, so shouldn't be taken within two hours of an iron supplement.