Look we don't know what her situation for the other kids was, but she was on birth control. The depo shot is one of the most effective forms of birth control, and because you only need to get it once every three months it has a much lower failure rate than condoms or the pill. Look I'm guessing she made some irresponsible choices in the past for sure, but I'd also be fucking pissed if I thought I finally had reliable birth control sorted out and was done having kids, and then I was pregnant anyway.