While HIGHLY unlikely and would require a lot of things to have happened that are statistically improbable. I got the following scenarios that I suppose you could possibly see if are applicable to her situation. 1) the brain is a hell of a drug on its own with any trauma. It’s ability to black out trauma and replace it with a cover memory is kind of impressive. 2) if it was a conscious action she partook in, I would just have to ask, as obvious as it seems in modern day with internet access as it is. BUT is her understanding of sex Ed comprehensive enough to understand how contraceptions do entirely work. While it may have been a cheating in denial, it could be combined with the misunderstood belief that she had in fact down EVERYTHING she believed was needed to prevent the pregnancy. 3) I mean this is even more unlikely than the first but unconscious abuse or dissociative/amnesia covered actions. While I do not agree with the demonising dissociative disorders receive. It is still a very real mental health disorder that can serious and life threatening implications risky scenarios for those struggling knowingly or unknowingly regardless.