I haven't been diagnosed with BDD however I did have extremely poor body image issues resulting in an eating disorder when I was a teen and thinking I was absolutely huge when I most definitely was not.
I still struggle with my body image however i do weigh a lot more now than i used to (eating issues seemed to almost flip into the opposite problem in my late 20's!). Pre-pregnancy i was doing slimming world and my BMI had gone down to 29 and I felt good that I was finally having a healthy attitude to food and nourishing my body, rather than seeing food as either the enemy or as something that I can fix my feelings with.
I have gained just over a stone at 32 weeks.
However all that is irrelevant I suppose as you're not asking about ED's etc, but BDD, although the 2 closely linked in my experience (although as I say, never diagnosed with BDD).
I have found my changing shape v difficult to cope with. I got v upset around 16-ish so weeks when my belly started to round out and my boobs grew 2/3 cup sizes and seemed to say overnight. I felt extremely unattractive and went back to covering myself up all the time and not looking in mirrors. I seemed to sort of come to terms with it a bit and started to feel a bit better and feel like I looked ok, but then any picture I'm in at the moment I honestly look twice the size of what I think I look like and then it confuses me because I feel like I dont know what I actually look like! I end up asking DP 'are my legs the same size as hers' etc. Not obsessively, only a couple of times literally just to try and gauge how 'big' I am because it's almost like my own image is blurred out and i cant see it accurately, but i can see other people perfectly fine. Hopefully that makes sense to you!
How i have tried to deal with it is making sure that i am eating healthy. If i know I'm eating healthy then i know realistically my body will not grow disproportionately than it should and my growing belly and thighs etc are 'normal' and 'healthy'. If my eating was out of hand, I would have more room to wonder if I really am getting 'too' big. Also, I know that once baby is here I can get more back to 'normal' and it's only temporary. I say 'more's back to normal because I'm aware that my body will not be exactly the same as before and I'm being realistic about this, but for me the problem is more about size rather than stretch marks etc. I also try to be compassionate towards myself and speak to myself in my head as I would to a friend who was struggling in the same way - berating myself, telling myself horrible things only serves to make the problem worse, so I try and imagine that I am talking to a friend and have empathy for myself (easier said than done and I only managed to do this after therapy!). If I'm having a bad day, I put clothes on I know I feel more comfortable in and avoid mirrors/pictures - I know that's not entirely healthy but if it helps me to get through that day then sod it, I can always work on the self empathy when I'm in a stronger position to!
Hope that's helpful and what you were looking for!