How to Prevent Pregnancy Weight from Melting Off -- Have Always Been Underweight

knottie25b33a08aa58e679

I'm not yet breastfeeding, due in 7 weeks. This is my 4th pregnancy and I have breastfed all of my other children successfully for 18 months each. I am a small woman and always have been. Started this pregnancy at 5'2" and 102lbs. I've always lost all of my pregnancy weight within a month or two of giving birth without having to do anything. I try to eat well, but can honestly eat whatever and however much I want to and it doesn't seem to do anything to my weight. My question is how can I prevent ALL of the weight from falling off? I'm 32 weeks and have gained nearly 40 pounds so far which is unusual. Normally I gain about 25 total. I am happy being a little heavier. It has boosted my self esteem tremendously and I don't feel quite so much like an adult trapped in a child's body anymore. I really don't want the weight to melt off like it has with previous pregnancies, but I don't know what to do to prevent that from happening. Any tips or advice?

Thanks!