Long story short: in my twenties I (40f) had a one night stand with this guy "Nathan" (now 41m), and got pregnant. I first thought of having an abortion, but Nathan had come from a conservative religious background (not Christian) and wanted the baby.
So we made a deal: I would give birth to the child and sign away my parental rights (my name is not even in her birth certificate, it's a possibility in my country), and he would pay all the medical expenses plus some extra for my troubles. That's what we did and didn't keep contact after his daughter was born.
A few weeks ago, a friend of mine who has the same religion as Nathan sent me a post from her temple's youth group (they were asking for donations for the homeless) and one of the girls from the picture looked a lot like me. I searched for her profile and found out that she had Nathan's surname and had the right age to be the baby I gave away.