Everyone has different comfort levels when it comes to choosing a doctor -- some prefer fancy credentials, some go on referrals alone, and some just want a soothing bedside manner. Your first pregnancy though is an emotional, exciting, nerve-racking time, filled with various tests, multiple appointments, and, inevitably, several "Is this normal?!" moments. And in the end, this man or woman will be the one bringing your child into the world, so you may be even more discerning about the OB that's right for you.
Without a doubt, age often comes into play. Many might want a doctor who's been-there, done-that and knows his way around an L&D room. Others might want a younger doc who's up on the latest technologies, still has that youthful exuberance, and may even be having a new baby themselves. As for me, I've always chosen doctors who are somewhere in the middle -- they still seem to get joy out of what they do, but they've done this many, many times before.