Just last night I had my first of the charlie horse cramps -- I stretch my legs frequently thru the night, always with my feet pointed. It immediately hurt and I remembered that I have to FLEX my feet or I get those cramps.
With my last pregnancy, my midwife told me to take my calcium at night for that. Prenatal vit's have almost no calcium, but lots of iron. So it means you have to remember to take the prenatal well before you take the calcium so it gets absorbed.
Potassium, calcium, sodium, and I think magnesium all have to be in balance. But it's anyone's guess what that balance is!