Hi There,
I have a 17.5 month old and she does the same thing. She probably started around 15 months conveniently just after her 15-month check up. I've read a lot of information and it appears that this is a normal process for some kids and can actually last for years. In the content I've read there really isn't any concern of autism unless a member of your family has this disorder. Some articles actually tote that your child may be more intelligent that the average child. And, some say that if you've done something different in their routine it could cause this type of outbreak. When she first started exhibiting these "skills" of head banging we were transitioning her from bottle at night and her nightly routine was getting shaken up with a few other variables. The largest portion of her head banging was only at night and we've since subsided by trying to keep to the same bedtime routines.
When she does exhibit the head banging, we try to monitor it and don't let our daughter do this continuously. Initially, she'll bang her head but we'll then go in and intervene and attend to her needs versus letting her continue. Again, it is often trying to find out what we did different in her routine and address it. That is how we are dealing with it. Her 18 month appointment is later this month and I intend to bring this topic up to her pediatrician. I'm not sure what more I can offer other than I am right there with you... it's rather scary to see your child act like this. Yet, from my perspective, she's developing all of her motor skills, continuing to be very happy and outgoing, and progressing as she should so I'm not as concerned as I once was.
One other thing I forgot to mention... when this was occurring at bedtime, we decided to start her routine a little earlier than we had been. Instead of 8:00, we bounced to 7:30. This also made a big difference and may be something worth trying.
I hope I've been able to help you out.
Take care,
K.