This is something I've noticed in quite a few kids and makes my heart ache whenever I witness it again. For about a week after twins are born, they sleep in the position they were when in the womb. Twins most common position when born is face first. This causes the first born to lay down first, then its twin lays down after resting its chin on the first Born's back. Ever walked in and realized you missed a whole birth?? Don't worry, just watch them. They'll tell you how it all went down. Lol.