Hey -
I have not read all of the answers, but I wanted to share my experience with you:
From about 6 months on, I knew something was "off" with my son. He made eye contact and he would laugh and smile, but something was just "off". He didn't make any sounds at all before 8 months and he also would not move around much - would just sit and stay in one spot. I knew he could hear, so I was thinking that it wasn't his hearing either. BUT, they did give him a hearing test. He responded to all of the noises they made, but apparently not quickly enough and he had to "search" for where the sounds were coming from. He definitely was NOT hearing as well as he should have been. Turns out that he had had so many ear infections that his hearing was impaired and so they gave him ear tubes. He did then start making sounds. However, something was STILL OFF.
Unfortunately no one would REALLY listen to me. They went through the motions. He was evaluated by the state at 12 months and they said that he was in the "normal" range for activities (honestly, the very low normal range, but they wouldn't do anything since he was in the normal range).
During this time, I did move twice, so we did have different pediatricians I was talking to. In our last move, the pediatrician seemed more receptive to what I was saying about him being "off". By this time, he was 18 months old and she flat out told me that they don't really evaluate kids for "offness" until they are around 18 months. So, he was finally at an age where they would start evaluating him. Fortunately for me, he was still speech delayed because of the hearing issues, so we started with speech therapy. And also thankfully for me, he went to an "all around" therapy center. The speech therapist worked with him for maybe two weeks (I was with him at each appointment) and at about 2 weeks into it, she buried a toy deep into a tub of rice. She said, "Can you find the cat in the bucket?" He completely ignored her. I thought to myself, "There is no WAY he is going to reach into a bucket of rice and dig around for a toy." I KNEW that wasn't going to happen. Absolutely no way. I think she knew this too, so she didn't press him to do it and she asked me if I wanted an occupational therapy eval. Thank GOD. That's really what I had been after the whole time I was trying to get help. So, finally we had the occupational therapy eval and they diagnosed him as having "sensory integration disorder".
Sensory integration disorder is one of the things that autistic kids ALSO have. So, things that may appear "autistic" to you could be sensory integration disorder. Google it and you will get a lot of information. Once he got the diagnosis, I read a book called "The out of sync child". In that book, the woman said that early intervention could make a "world of difference". I asked our therapist what that meant and she said that early intervention could "re-wire" the brain and teach a child how to respond differently.
In our case, early intervention and therapy was a MIRACLE. My son is now what I would say 99.9% better. He's now 9 and no one would ever guess that he had any problems. The only thing that is still "iffy" is bike riding/balance issues.
So, basically, I am telling you all of this because it might be hearing issues with your daughter, it might be nothing, it might NOT be autism, it might be sensory stuff. There are a LOT of MIGHT be's. My best advice is to follow your gut and go and search for answers. If your doctors continue to not agree with your gut, then get another opinion.
Now, in retrospect, I can understand why the doctors kind of waited until 18 months. There is a lot you can see at 18 months that you cannot see at 6 months. But, there is also a LOT you can do to help a 12 month old that would have helped my son sooner.
Good luck!
L.