Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about your pup and her possible history of a pelvis fracture and her ability to deliver puppies naturally.
Her ability to deliver the pups will depend upon whether her pelvic canal was narrowed with her injury and how big the puppies are at birth, especially head size. Some normal Chihuahuas need C-sections because their puppies tend to have very large heads in proportion to their body size and the head cannot fit through the mom's pelvic canal.
When was she bred?
If it was very recently and you cannot afford a c-section then spaying her now may be the best thing for her and you. Whether she is pregnant now or not spaying her will prevent future pregnancies and avoid this worry entirely in the future. Cost of surgery now will be more then a normal spay, but much cheaper then a C-section.
If she is already far along, or you cannot morally accept that she may be pregnant and so a spay would be ending the pup's lives, then your best bet would be to have radiographs taken when she is within a week of delivery to see if these pups will fit and what sort of injuries she has. Taking radiographs sooner then that will allow you to see her injuries, but won't let you estimate whether the pups will fit as they will be much smaller the earlier she is in her pregnancy. We cannot see the pup's skeletons, and thus head size, before her 45th to 50th day of pregnancy as their skeletons aren't calcified before that. If her pelvic canal is very narrow or the pups are very large then a scheduled C-section is cheaper then an emergency one, so it is best to have some time to plan ahead. Knowing that she likely has a broken pelvis makes it less likely she will be able to deliver normally, but rarely they can.
It won't be any more expensive now to do these things then if you knew her history as we would need to do the same tests to determine the size of her pups and her pelvic canal no matter what.
Best of luck with your girl.