About 1 in 59 children are diagnosed with autism, according to an estimate by the CDC, a number that has grown significantly over time.
The diagnosis can be a source of severe emotional and financial stress on families. A 2014 study found that it costs $2.4 million to support a person with autism throughout the course of his or her life.
"I wake up every day and not only fear for my own child but for all of these kids," McCarthy, 45, said.
The actress and advocate has also drawn controversy by promoting the idea that vaccines may be linked to increased incidence of autism, an argument that has been scientifically unsubstantiated. (In the past decade, researchers have come a long way in narrowing down which factors do make autism more likely to develop.)
McCarthy talked to CNBC during trading firm BTIG's 16th annual Commissions for Charity Day, when celebrities act as guest traders and raise money for a range of causes.
With the help of other big names, including Bill Clinton and Shaquille O'Neal, BTIG raised more than $5 million.
McCarthy said she would love to see more celebrities use their public platform for good. "We already have the gratitude of having a blessed life for doing what we love so why not be more of service in this lifetime and find something that people desperately need and fill it," she said.
"Don't wait to have the cause happen to you in your life, go ahead and pay it forward."
For Mother's Day, McCarthy said she is planning on a day with Evan, now a teenager.
"My son thinks that Mother's Day means that he gets to tell me whatever he wants to do so I let him have that rule," she said. "I make it his day too."
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