How is a baby born with the sickle cell trait when neither parents has it?

New mutation: Sickle cell trait is almost always caused by a gene passed down from one parent. The parent may not even know he or she has it. Rarely, a new mutation can also occur-- a child's gene changes on its own, meaning that they didn't get it from a parent. I could not find the exact rate of these new mutations. It's rare, but it can at least theoretically happen.

New Mutation???: In theory, a new mutation where the instruction is changed in the gene sometime after conception. I could find no data on frequency of such a possibility. It is more likely one of the parents carries the gene and is not aware of it. Trait seldom causes any issues and ~1/10 afro-americans carry the gene. Neither parent needs to have full blown ss disease to pass trait.