Autism diagnoses continue to rise, in part because the definition was expanded to include the "autism spectrum." Nonetheless, it is a heart-breaking diagnosis for a parent to hear. Researchers are trying to find out the cause, and that is leading them to explore pre-birth exposure to drugs and chemicals.
It has long been suspected that mercury in the form of thimerosal in vaccines was a cause. Thimerosal was removed from vaccines in 2001, except for the flu vaccine, and still the autism case numbers rise. That doesn't mean thimerosal is vindicated--just that there are undoubtedly more contributing factors than mercury alone.
Another suspected contributor may be drugs that alter serotonin levels, such as SSRIs, a popular class of antidepressants. This class includes fluoxetine, sertraline, and paroxetine. The studies are only in animals so far, but researchers have found changes in brain function of the babies born to mothers exposed to high levels of serotonin. SSRIs are popularly prescribed for pregnant women with depression because it has long been thought that they are the safest in pregnancy. That thought may change as more information emerges.
If you are pregnant and taking an SSRI, I don't want you to interpret this as recommendation that you should suddenly stop taking the medication. Please talk to your doctor before you start or stop any medication during pregnancy. Untreated depression in pregnancy has been linked to premature birth and low birth weight, both of which are serious risks to the baby. Everything has a balance of risks and benefits.
Exposure to rubella or cytomegalovirus has been linked to autism. Make sure your vaccinations are up to date before you get pregnant (see previous article on vaccines).
You can search and find a lot of information on suspected causes of autism. I think you can drive yourself crazy with worrying about whether something you ate or the body lotion you use could be a problem. And during pregnancy, I can attest, your fears can get pretty extreme! My theory is to eat healthy, unprocessed foods and use products that have few chemicals, like baking soda to clean rather than harsh cleaners. Start this before you get pregnant!