Being limited by 300 characters, I had deleted a much longer response acknowledging how the law may have passed to make slavery illegal, that the racist oppression and abuses continued in practice if not in name. I admit to reducing that to a much shorter, over simplified "power dichotomy".
The fact of the matter is, although horrific slavery practices continued, and the 1930s were no incredible highlight of freedom for Black and African Americans, we have no idea based on the single post how she became pregnant beyond the information provided unless you have a source specific to her.
But again, the sick power dichotomies I speak of between an adult and a 12 yo don't exclude a white man in a position of authority whether explicitly defined or not. It's a possibility. Unfortunately we just don't know.