Honestly I think that between interviewing pregnant and interviewing with a 9 month-1 year old it's really a toss-up. If you interview pregnant they will probably realize that you are due before you begin residency. If you take a year off then obviously you will be more "settled" when residency begins, but they might wonder if you will be looking for child #2 somewhere down the line. Bottom line: you are going to have a child and you ultimately want to be at a place that will be friendly to that so it probably doesn't matter when you interview.
With that said, would you rather take a year and start residency when your child was over a year old or would you rather start residency with an infant?