State investigators say the hospital was unable to supply any records to prove Rivas gave consent.
Hospital officials say they got approval over the phone.
Rivas' son, Nathan, is now 3 years old with the mental capacity of an infant and needs feeding and breathing tubes and nursing care. According to Tuesday's court testimony, Nathan's nursing care costs approximately $1,100 per month.
Rivas believes the boy was brain-damaged by a bad reaction to anesthesia. The hospital says the child's complications were caused by being born four months premature.
AP and CNS contributed to this report.