The advice I give everyone is this: if you are concerned about a link between x and y (in this case, ultrasounds and autism), try to find the original research article. Google Scholar allows you to search for articles, and if a colleague or friend told you about a study, ask for them to share it with you.
You may already know this, but you'll want to make sure that the study is peer-reviewed and not simply summarized on a website. You'll want to pay attention to how many people were in the study, and who those people were. You'll also want to pay attention to who conducted the study. As a scientist, I'd say that the only way to determine whether ultrasounds are (or are not) linked to autism is to assign a large number of mothers to either have (1) many or (2) few ultrasounds. This would be the only way I could think of to show a that ultrasounds do (or don't) cause autism. As others have noted, if you just look at the *correlation* between the number of ultrasounds and autism, you'll likely find a relation, because higher risk patients are both more likely to get ultrasounds and have a child with autism (in other words, knowing how many ultrasounds someone had might help you *predict* if a child will have autism, but that's not because ultrasounds *caused* autism).
If you aren't familiar with or comfortable with reading scientific articles from journals, feel free to post or PM me, and if you send me the article (or a citation), I'd be happy to try to help you evaluate the article yourself.
Cheers to all!