Joe and Mary Thompson had agreed to adopt Emily before her birth in 1999, and it never occurred to them to back out when she was born with spina bifida. But that same year, their residential remodeling business in Overland Park, Kan., went under, prompting job changes that left the family searching for health coverage with a child who was uninsurable.

The insurers were willing to cover the Thompsons and their older daughter, but not Emily, who was later discovered to have mild autism as well, or her 13-year-old brother, who had a diagnosis of attention deficit disorder.

Starting Thursday, the insurers will not be able to do that, as the new health care law prohibits them from denying coverage to children under 19 because of pre-existing health conditions. In 2014, the change will extend to people of all ages.

“It really is a pinch-me moment,” said Mrs. Thompson, who is 50. “Could it possibly be, after all these years of fighting and jumping through hoops and trying to find the right place to help us out, that she could just be put on our policy with her sister and not be discriminated against any more?”