Don't know if this is true but one of the midwives i saw during my PG with DD said that you could tell if you had a good birthing pelvis by feeling with your fingers.

You basically put the first 3 fingers of your hand against your vulva (not necessarily IN the vagina though) and pull upwards, until all 3 are hard up against the underneath of the pubic arch. She said if your fingers stay in a straight line your baby will "fall out" (was true for me anyway! though i had a bit of an OP labour to begin with), if your middle finger sits a little higher between the other 2 you should make sure you're on hands and knees to deliver and if your middle finger goes so high that your other 2 are almost touching then you can expect a longer 2nd stage but, she stressed, a vaginal birth is still very possible with patience and support.

Bx