The best time to ship your dog would be 8 weeks after she gives birth, to provide the optimum safety of the mother and the puppies.
However, shipping during very hot or cold months is less desirable than other times of the year.
If she must be shipped prior to giving birth, than she should probably be shipped ASAP, but there is always a risk during pregnancy. I personally, would not want to risk my dog under these circumstances.
Having her shipped in the cabin would be the safest way. Service dogs, such as therapy dogs, seeing eye dogs and other guide dogs, and dogs for the deaf, are allowed to be shipped in the cabin, with their human.
I guess pregnancy in dogs is not health related...