My 5yo DD was recently diagnosed with ASD (Aspergers). I had to to go private to get the diagnosis though.
The main red flags for us were her emotional volatility, she would burst into tears and run away screaming at the smalled of things (she's demand avoidant). She also could not cope with any change, any change to her routine would result in horrendous behaviour and a meltdown.
That was all when she was 2 years old.
By the time she was 3/4 the above continued but her sensory issues started becoming more apparent and we started linking her behaviour up. She is a huge chewer and will chew anything she can ger her hands on.
We started noticing she was struggling socially with her peers, always a follower and would never initiate. She would literally stand on the edge of a circle jumping up and down but no one noticed her.
She also has selective mutism and anxiety and will not speak to any strangers, or even seeing a close family member out of context. So normally we see her grandma at her house, and bumping into her at the shops my DD wouldn't look at her or speak.
Her eye contact is actually ok, except when in a situation as described above and she will refuse eye contact.
DD also masks spectacularly, school see no problem with her and she has chameleon like tendancies. School are the reason we had to go private as the SENCo repeatedly told the Paed there was nothing wrong with her so we were discharged.
Sorry that was very long
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Ultimately you are her parent, and we just know.
Have a look at the NAS website. Tony Attwood has written lots on girls with Aspergers and I can relate to a lot of the stuff he has written.