I had cramps in my calves with both of my other pregnancies from the second trimester onwards. I can't remember where I heard it, but it is important to stretch the cramp, not squash it up further so I found it really useful to stretch my calves by pointing my toes back towards me as soon as I woke with the cramp and found that this did the trick. The worse thing you can do for the pain is to point your toes away from you. I did this once and the cramp was so bad it took a while to get rid of it. I think I have described it properly anyway - if you pull your toes towards you then you should feel the calf muscle stretch. I found this useful for me, hope it is useful for you too.