Not on the back, but on the side (they all have little bumps on their backs lol). Right behind his/her front legs. Like the armpit area. If it's a male you'll see a little white bump on either side. Check if they're both male / female. If that big one is a male obviously he can't be pregnant but also if you have 2 males or 2 females it can't be pregnant (b/c who would impregnate her :lol