I was once told that most Aboriginal women give birth to posterier babies, and possibly the shape of their pelvis is the reason behind this. (I don't know how true this is, but it was a midwife I respect who spent a lot of time working with Aboriginal ladies and she said they thought it was strange that a baby would be born not looking at their mother's face.)

Anyway my point it, perhaps your pelvis is shaped in such a way that your babies prefer this position? I don't know how you would find this out though. Are you planning another baby? If you are perhaps a chiropractor could help you with your pelvis prior to labour to help keep bub in the most comfortable (for you) position.

T
xx