How do they explain the baby's fast growth to Bella's parents in The Twilight Saga Breaking Dawn?

Bella's parents actually don't know anything about what goes on with Bella in that entire part of the book. They, and Bella's friends, think she contracted some rare disease and is in a hospital somewhere across the country. To "back up" their story they gave people an incorrect number that they were supposed to be able to use to contact Bella.

So, because her parents didn't even know she was pregnant, they didn't know about the baby, therefore they didn't now it was growing crazily fast.