See below. 1-2% incidence in the US for autism. 6% incidence for the full spectrum of PDD. Check my health tips on PDD. See http://www.Nimh.Nih.Gov/health/topics/autism-spectrum-disorders-pervasive-developmental-disorders/index.Shtml or http://www.Nimh.Nih.Gov/health/publications/a-parents-guide-to-autism-spectrum-disorder/what-are-the-symptoms-of-asd.Shtml.
Many Factors. The chance of a child having autism depends on family history. The chance of having a first child is overall about 1 in 110. If you have already have had a child with autism, the risk goes up considerably. The risk also depend on ethnicity, being higher in non-hispanic whites compared to other ethnicities.
Uncommon. Despite a claim of a high incidence of autism, the likelihood of a child being born with autism is still rare. The number of children diagnosed with autism has been inflated by re defining other diagnoses, for reasons I do not fully understand. Part of the problem is including children who do not have autism into the category of autism, in order to provide services to them.
Stats. The general odds currently stand at 1 out of every 68 children born for autism spectrum for the latest year of prevalence data collected which was 2010. Excellent article by CDC here: http://www.cdc.gov/ncbddd/autism/data.html I must clarify this is not specific for autistic disorder- the most severe type. But will do some digging and will try to provide an answer later. Take care.