Think of your pelvis as a gate. The purpose of the hormone Relaxin is to make your pelvis open right up during birth to let the baby through, that's why there is a gap at the front of it. If you lay down you should be able to feel a slight indent in your pelvis and that's what opens up. You cannot assume before you go into labour that you cannot vaginally birth your baby just by looking at you. Very few women truly have CPD (where the pelvis is genuinely too small or of an odd shape that is not favourable for birth). There is a thread on here with a link to a site where some really tiny women give birth to average size babies vaginally - there is a lot of graphic photos, but well worth having a look to put it into perspective for you. I will find te link and post it here for you.

My own Mum is a tiny woman with very small hips, but she was able to vaginally birth all of her babies and all but one of us was well over 4kg, with the biggest being 5kg (the smallest bub was nearly 8.5pds, but he was prem) and to look at her you would never have thought she'd have gotten babies as big as us out, but she did.