The closest to a definite sign of autism might have been the absence of babbling.
When do babies start to babble usually? I don't know for sure, I'm guessing at roughly half a year, but I was eerily quiet all the time until I was a year old, not making expressive sounds, not trying to imitate speech.
As for a not-so-definite signs of autism that I guess I might have had, an early lack of facial expression.
Most prominent on almost all my photos is the same expression, one of wide eyes and a blank face. People think it's very "cute".
Another thing is walking. I wonder if it is connected to my autism.
I learnt to stand and walk so early that the possible consequences for my joints (claiming I was walking too early and needed to be stopped, suggesting to restrain me even to prevent me from it entirely!) worried the doctors.
I was brought to see one after the next albeit nothing was found wrong with me until I was finally at an age at which walking is, for some reason, considered "early but somewhat normal".
I am most thankful to have avoided some hideous manner of restraint to keep me from walking and moving at all.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett