Leg cramps are common in pregnancy. A recent Cochrane review found moderated quality evidence in papers more than 20 years old.1
They concluded that the best evidence was for Magnesium tablets either with a lactate or citrate base taken as 125 mg man or 250 mg nocte. The number needed to treat was about 3.
There was old evidence that supplemental sodium may help but felt that community intake of sodium has increased that the findings may no longer be relevant.
They also concluded that extra calcium was not effective, and while multivitamins were also effective, they could not tell which components were the effective ones.