I think I'd offer an excuse and make it known that I'm sorry, I realize it's lame, but.....(insert excuse) and "I've been so remiss, but I'd love to come see the baby."
Thing is, I think most pregnant women would not understand the phobia thing. Myself, I feel a slightly sick feeling when I see a pregnant belly and I don't know why. I know it's natural, I know it's not "sickening" but my reaction is what it is. It's very slight so it does not prevent me from interacting with pregnant women, but when I tried to explain it to a friend (who was not and had never been pregnant) she seemed a little shocked and horrified. She didn't seem to understand that it was just an irrational gut reaction that I knew was irrational. She took it as "I find pregnancy sickening" which I don't really. But the sight of the belly does trigger a reaction, ever so slightly. I don't even really expect other posters here to understand.
Which is why I'd go with the excuse, as your friend may not really like hearing that you have a phobia around pregnant women.