The most realistic aspect of Bella's pregnancy would have to be the cravings and nausea she experiences. At one point during the honeymoon, she wakes in the middle of the night and makes homemade fried chicken. Shortly thereafter she experiences nausea, consistent with morning sickness. Cravings continue throughout and while much of her pregnancy is certainly fictionalized due to circumstances, it could be said that it is as realistic as possible considering she is carrying a vampire baby.
For instance, the child would be strong, which could account for the breaking of bones. Additionally, the amount of strain her body would suffer due to a gestation that occurred twice as quickly would certainly weaken a human body, as we are not built to adjust as quickly as her pregnancy developed.
There are some cases in existence where pregnant women have endured broken ribs, but usually this is due to mitigating circumstances, such as a cold or weak bones. As far as I know, there are no studies, nor are there enough cases to have cause of any, on this particular topic.