I'm the youngest of four kids. (Mom said she wanted six.) When I was born, my siblings were 17, 16, and 10. Noting the large age gap, I once asked my parents why they had another kid that late in life. With a sly grin, Dad said, "You were a forgotten trip to the drug store."
I know he was joking, for three reasons. First, my parents were absolute devout Catholics, and at that time, the church opposed birth control under any circumstances. Second, my dad worked at a drug store! Third, he had a great sense of humor. While he usually kept it G-rated, once in a while he'd go for a little touch of shock factor.