If I get it this time I'll probably do it after the birth. I'm not anti vaccine in general but I do think more things cross the placenta than we realize.
I got it right after mine was born.
I got the tdap, it does help the newborn develop some kind of defense against the illness but totally get where your coming from, keeping it natural :)
It protects against diphtheria, tetanus, and most importantly purtussis (not really sure on spelling!) which can be deadly for a new baby. Since they don't get the first of those shots until months after birth, it's recommended. However, there haven't been sufficient studies about the effect of vaccines in pregnancy for me to feel comfortable taking the risk. I'd rather risk the illness.
Get informed and talk to your doctor more before deciding one way or another. Just because it doesn't feel right to me doesn't mean it isn't right for you.