If I understand correctly, taking a life science approach to this, it's not 33%. I didn't care enough to pay attention back in 6th grade when we talked about sex (I lit my binder on fire using a magnifing glass one day, though. Teacher yelled at me), but I think there's a chance somewhat less than 50% for a girl to be born, and a bit more than 50% for a guy, and that it doesn't change at all after the first child.