I didn't show any AS symptoms when I was 3 or under. I was like a typical baby and toddler, reaching all the milestones around the average stages, and I caused no concern for my parents (and I had an older brother, so they already had experienced having a normal baby). I also didn't cause any concern for the carers at preschool either. I interacted with the other children (age-appropriately). I may have been a bit shy but a small child is entitled to be shy, whether they're Aspie or not. Not all children are exactly the same, but I didn't give off any noticeable differences. My Aspie traits must have just been mixed in with typical baby/toddler traits. I even was smiling in a lot of photos of me as a baby, as young as 6 months or whatever, and I always liked being played with.
It wasn't until I started school when I caused concern for teachers. I was OK at home still, but at school I did display behaviour that was noticeably different to typical behaviour of a 4 or 5-year-old. But because I was only mild, they didn't quite know what was wrong. I was assessed a lot of times, until I finally received an official diagnosis at age 8 and a half.
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Female