Coming back to clothing, I would wear nice slacks with a blouse. I was in the same situation just 3 months ago. My interview was a couple of days after my due date, and it couldn't be changed since I applied early decision (only one interview date). That's what I planned on wearing if still pregnant. Luckily, baby came 2 weeks early. Sleep deprivation was so brutal I don't know how I was able to talk coherently to my interviewers, and I won't even talk about engorgement, but I got in!
Ideally you would buy an interview suit, but how many times are you going to wear it? Even if you have several interviews, it will be hard to wear the same suit since your belly will take proportions you never imagined it could. If you're OBVIOUSLY pregnant, no one is going to hold against you the fact that you are not wearing a suit. As long as you look professional (like I said, black pants and a nice maternity blouse). You will save a few hundred dollars that you can spend at Babies R' Us instead! Some girls didn't wear a suit at my interview and they didn't even have the pregnancy excuse. As for me, I miraculously was able to fit in my suit just after giving birth, but boy it was tight.
There's a thread in the non-trad forum about this. You may want to look for it. You can PM me too if you want.
Good luck and congrats! You will have so much fun with your baby.