I would say, you would be the best judge of that. If they were truely pregnant and now are thin, I would have to agree that they dropped their fry.
If you have a heavily planted tank, I like to use a flash light at night to look for the fry. In the mean time, make sure you are adding some crushed flake food or whatever you normally use, during feeding time.
Of course, my filter is also a good indication of fry. LOL. I always find some in there after birth.
And I'm sure you already know this, put there is also a chance that the mothers and/or father will eat them.
BTW, if you have a planted tank and want to clean the gravel, look into getting some MTS. They do a GREAT job. They wouldn't clean up everything, but a quick surface skim and you will be all done.