It is not super common to impregnate on first offense, especially if you were her first child. You can count the days backwards from your birthday to see when it happened. If you were the first child, you may have been a day or few late.
Growing up, I found it funny how many of my friends happened to be born in the first week of September… Happy New Years. There is often, not always, but often some correlated reason why they were free to screw around too much.