I had a similar problem before it all went wrong. Overweight with SPD. The thing about eating is to have the nutrients for 2 not the volume. So that meant no junk food, reducing refined sugar and wheat foods and increasing things like brown rice, quinoa (high protein & calcium), fresh vegies etc. I used the Brewer diet for tips on what to eat as well. Good nutrition is essential for placenta health as well and the Brewer diet is good for that. There are some great 'superfoods' like chia and quinoa that can be added to porridge etc. A good book on pregnancy nutrition is Feeding the Bump.

Wrt exercise, sitting on a fitball using hand weights, clinical pilates & aqua mums classes, all adjusted for having SPD. At the end of my pregnancy with DS1 they only thing I could cope with was gentle hydro sessions. I gained a total of 8kg for that whole pregnancy.