How many calories should I take in daily?

I have been on an 1200-1300 calorie diet and 1-2 hours of exercise in a day to lose weight at which I was losing 3 pounds a week. I am an obese lady w/ a lot of weight to lose. I have just learned I am pregnant and am wondering how many calories can I take in at this time and how much exercise so as not to gain any weight in my 1st trimester. I know I can gain 15 lbs. safely in the pregnancy so I think this early on I don't need to gain any. Also I have a history of gestational diabetes and high blood pressure w/ my pregnancies also I had a c-section in my last pregnancy. So I was hoping to continue some kind of regime so as not to gain a lot of weight and keep the high risk pregnancy conditions at bay and to have a safe vaginal birth. Can anyone give me a precise amount of calories I should have in a day and how much/kind of exercise can I do w/o harming myself or the baby. Thanks to all for any advice given.