Yes, you put it better than me - such authority of female bodies.
I think she knew towards the end but maybe at the start, before the abortion cut off, she could have genuinely believed that the spotting she experienced was a period or maybe she didn’t experience any bleeding but tried to but it to the back of her mind. I mean she was only 14 when she got pregnant and 15 when she gave birth, she was still a child. Some much older women don’t know fully how their bodies work.
When I was about her age my periods were incredibly erratic, I sometimes wouldn’t have one for weeks. Even as an adult they are like that, I didn’t think twice about not having a period for ages and I was actually 4 months pregnant (the eventual morning sickness gave the game away but again not everyone gets that!
(ironically during the later stages of pregnancy I had false periods!)
No two women are the same, like you have all said!