Do remember that pelvises are not fixed, theyare a series of bones which are attached by ligaments which stretch, and with the aid of the hormone relaxin which we secrete while we are pregnant, everything in our bodies can stretch an amazing amount.
Our pelvis is designed to open the exact right amount to let our babies out, it is usually only because of physical damage eg a car accident, or having had rickets or something which means our pelvises wont perform.
One of the biggest factors in giving birth is positioning, and being able to move freely while in labour, as our babies corkscrew their ways out of our bodies, so we need to be able to facilitate this by squatting, putting our legs up on furniture, being on hands and knees or whatever our bodies tell us to do at the time, and often we will need to change position many times during labour.
A lot of Obs like women to birth on their backs, which does not facilitate birth at all, thus their terrifyingly high c/s rate (among other reasons)
Our bodies know how to birth babies, we only need the space, time and security to be able to listen, and to respond.
The Pink Kit is an amazing resource which gives great information about the way our bodies work to give birth, and it explains all about our amazing stretchy pelvises.
The only way to know if a baby will fit through your pelvis and be born is to give birth, as every baby is different, and it is a dynamic process which a static x-ray or scan simply cannot predict.
Absolute belief in your body is one of the biggest predictors of a successful vaginal birth.