Her kid wasn’t orginally told that she was his mother, but it doesn’t say she wasn’t told. Whether a five-year-old could understand the difference as we would understand it is debatable. It must have been obvious to her that the boy came out of her body, but it would be easy to confuse a 5-year-old that it could still be her sibling, not her child. A horrific experience for her anyway.
From the same page:
And also from the same page, totally OT but fascinating nonetheless.