Prenatal boa development?

My friend thought her RTB might be gravid, so we x-rayed her today and saw at least one confirmed skeleton.

I know with dogs you don't see radio-opaque ossification until around day 45 of gestation, but I know absolutely nothing about boas. Would anybody who's done this care to hazard a guess at when she might be due? Can we measure the length of the fetus on film and guess at how close it is to birth size?
This friend does not keep accurate records, does not follow standard breeding practices, and well...doesn't take advice well. So any information I can use to help her (and the snake) would be useful.