Hi there,
I'm sorry that I can't help with the PND or the weight issues, but I can try and help with the shock and panic that you are feeling - maybe!
I found myself VERY unexpectedly pregnant with our fourth about 17 weeks ago, and I was in completely turmoil and fell into what I feel was a depressive state. In fact when I found out I was literally hysterical - physically shaking, hyperventilating, floods of tears, the works. I was adamant that I didn't want another as the three that I have are a handful enough and in fact DH was booked in for a vasectomy (which he has now had). Termination is not an option for me on religious grounds and so I have had to face the fact that I/we are having another.
I went straight to my GP and she referred me for counselling. I have had four sessions and it is really helping me to get myself in a positive frame of mind about it. I've never had any sort of counselling before, and I was somewhat sceptical, but it has really made a difference. I am sure that if you talk to your GP about your PND and weight issues that there will be a way to help you with those too - as someone said, you can manage your diet whilst you're pregnant without harming the baby.
We are going to struggle financially with four, despite on paper having a very decent income - I will have to give up my job because I am barely breaking even with 3 of them, and can't think that will change with another one to pay childcare for, so not only am I struggling with the thought of having to go all the way back to the beginning AGAIN, but having to give up my professional career (at least for the moment).
You and DH have to try and pull together on this - I have a very supportive (if very shocked) DH which has made all the difference - try and sit down and talk it through rationally, without blaming each other and see what conclusions you come to. Yes it's a huge shock, but if you really want to keep the baby, as long as you pull together, you can make it work somehow. I have a small gap between my first two (15 months) it is doable, even if it's bloody hard work at the time!
I don't know if I'm making much sense (it is 4.50 in the morning, and I've been up since 3am - my brother's girlfriend died of terminal cancer yesterday and as I couldn't sleep, I've been writing letters to the family), but I did want you to know that you aren't alone - but that looking at your post now, and knowing that's where I was 15 or so weeks ago, and knowing how much better I am feeling now, I know that you can get here too - with help and courage.