ok. I hear a lot of women ask about this. I think there is a lot of misinformation out there.. I studied human osteology for a long time, and its VERY rare for a woman to be literally 'too small' to give birth naturally. Now baby can get stuck - thats one thing - but to literally have too small of a pelvis is incredibly rare. The reason being is that your body releases a hormone called relaxin. It causes all the connective tissue in your joints to relax so that they can shift during labour. Your pelvis is made up of several bones and joints. Your hips will widen and the pubic symphisis will soften so that the 'bowl' like opening will get even wider.
Also FYI there are certain positions you can push in to optimize how wide your pelvis can get (on you back is the worst for keeping your pelvis small). I have heard than on your knees and side can help with making it larger. I recommend doing some research regarding pelvis size. I think you'll be surprised that there is some things you can do to help your situation if you are concerned about being 'small'.