Why is this so common in pregnancy?

The various theories regarding the causes of leg cramps in pregnancy include deficiency of vitamin B, calcium, or salt. The treatment would be appropriate dietary supplementation. Discuss the problem with your midwife or GP before taking any supplements. There are also homeopathic remedies which may help, but seek the advice of a practitioner experienced in dealing with pregnancy do not just buy them over the counter.

Night cramps can also be caused by restricted blood flow to the leg muscles. It can help to have a gentle leg massage to stimulate the circulation before you go to sleep. Lying on your side with a pillow under your bump and between your legs can also help to relieve the pressure of a heavy uterus. Elevating the end of the bed slightly is another suggestion.

How can I stop it once it has started?

Stretching the muscle out is the quickest way to ease the painful spasm. Dorsiflection of the foot [i.e. bending your foot upwards towards the shins] with a straight leg, is the way to relieve cramp in the calf or hamstring muscle.

If you have somebody with you, they can help by supporting the leg and foot. They may also need to massage the muscles afterwards to relieve any residual tension. If you are on your own, you can try pushing your foot and leg against the wall, door or any other solid surface until the spasm subsides.

If you need to change position in bed or want to stretch your legs out, always try to do so with your foot dorsiflexed as described above. Never point your toes downwards!