Weight gain

I have two beautiful daughters and would love another child we're currently tracking and we will see if it happens. My question though is to those who have maintained a healthy weight during pregnancy, how did you do it? With my first I put on 4 stone my 2nd was 3 if I did fall pregnant again I do not want to be putting on any excess weight I've worked so hard to get to were I am today and I'm still classed as over weight with a BMI of 25.4 I've lost 2.5 stone to get to that BMI in the last year. For my physical health I can't afford to put any more than the essential weight during pregnancy but I know also for my mental health I can't afford to put any on (I have suffered with an eating disorder in my teenage years)