FYI, what you and everyone should know as a breeder..... IF a girl/Jill absorbed/aborted her last litter, then you need to be testing her progesterone as this pregnancy goes forward throughout the 63 days gestation. Then you will know if she does indeed drop progestrone level prematurely causing her to abort. (Has nothing to do with intending to supplement) Progesterone should/could have been done post her last heat cycle to know more about her maintaining progesterone for future pregnancy potential success.
As far as supplementing progesterone, as with any pregnancy, anything can happen and it should not be done without a great deal of thought and research. However, there are numerous cases of supplementation 'when it was needed' (careful not to supplement when not needed) to maintain pregnancy, producing healthy, normal puppies, who went on to produce healthy puppies for generations. Yes, not a lot of data on it, but noted Repro. Specialists use it regularly.
Good Luck to Jill!
Oh, and edited to add that progesterone testing can be done at the end of a normal pregnancy to better gauge when the delivery will happen. Progesterone drops naturally to induce labor.