You should aim for half your body weight in ounces. So 200 lbs= 100 ounces. That goes for when you're not pregnant too.
As much as you can, especially in this heat! (if it's hot by you). I agree with irishlove1987, at least 8 to 10 glasses of water a day. It's very important to stay hydrated :).
It should be 8 to 10 8oz glasses of water a day.