How to Keep in the correct weight Range During Pregnancy

I am 5 weeks along and I am still carrying 20 lbs. from my first pregnancy. I am only to gain 15 to 20 lbs. during this one. I seems to be packing on weight and I'm not sure why. I was 175lbs. before getting pregnant and I really haven't changed my eating habits and I'm at 179lbs. I have been drinking a half a gallon of water a day as well. I just am looking for any suggestions for staying in the weight range I am suppose to and a healthy way do do it. I know I'm not suppose to diet but I need some pointers on how now to gain 50 lbs. like in my first pregnancy. Thanks!