As JoG said, I had problems from the start. I had spotting for 2 weeks right around the 12 week stage, thought I was going to lose him (doctor did too). Lots of nausea etc. Had high blood pressure and swelling for the last 3 months, had to spend the last 2 weeks sitting with feet up doing nothing. Induced labor that lasted 29 hours and ended with a c-section. He was actually a pretty good baby tho, slept right throught he night from birth, no colic. Had to wean him to a bottle at 4 months tho and he didn't like that. I did notice little things that were different all along though, he didn't like to interact with other kids, didn't talk except for a few words until he was past 2 and then it was pretty much bang full sentences, didn't handle transitions well. In preschool (3-4), the teacher would tell me he preferred to go off and do his own thing, and she didn't push him to join the kids, I remember one almost meltdown when another kid sat in "his" spot (no assigned spots) and he just couldn't handle it. Grade one (6) he started having more problems, teachers would tell me we needed to socialize him more (then they'd keep him in at recess and lunch to do work he hadn't finished - go figure). Grade two (7) it pretty much exploded. He started having meltdowns at school, some at home, and the whole road to figuring out what was going on started then (therapist, psychiatrist, behavior programs for school etc.)
I think 5-7 is a critical time, because that is when more is expected of them, and their not ready/able to handle the expectations.