What can I do in pregnancy to help prevent autism?

Do Not Know. The causes of autistic spectrum disorder are unknown, thus prevention is also unknown.
No data. We really do not know what causes autism. Part of the rise in autism may be due to better recognition and more frequent diagnosis rather than a true increase in the number of cases.
Get your "flu" shot, Not flu-mist, when your OB okays it, usually 2nd trimester, if you didn't get one this year & try to avoid people with acute viral illnesses. Don't drink alcohol, smoke or use drugs. Take your folate (folic acid) to prevent neural tube defects. Maintain a healthy diet to prevent excessive weight gain, high blood pressure, gesrational diabetes & follow your ob's instructions to lower risk of premature delivery.
May be too late. Recent data suggest over half of these are due to misinformation encoded in the DNA that would be present in the egg or sperm before conception. How they happened is a mystery, but it points out the value of avoiding toxic exposures during every day of your life.Avoiding drugs/alcohol/illness are things you can do to avoid other issues, but this one may not be preventable with today's technology.