Cass, definitely talk to someone if you feel she isn't developing right but I would tend to agree with sangres.

From my own experience, none of my kids were babblers at that age or would go near strangers. She might just be of a different nature to your other kids - I know I am always surprised at how different my third is.

I am pretty sure that A is delayed from the chronic sleep restriction as a baby - she was always red eyed and grizzly, staring blankly and not interactive, and she has developed slowly. I totally know what you mean about them seeming just dandy at home, then you see them next to a kid the same age and the difference in ability hurts she is getting there though. It seems to be snowballing now and I really hope she is at age level development by school start next year.