dougieee
Songster
Normally you could see them up until the chick has grown to a decent size. Then you look for movement instead.. I have seen them on day 14 of candleing. But mostly at the beginning
Ive never had any late quitters come to think of it, i get early quitters that blood ring in the first 7 days which i assume is genetic and the embreyo just doesnt fully form and then occassionally DIS fully grown.
I guess that as long as your bator doesnt play up once the embreyo is fully formed the growth stage in the middle is pretty uneventful until it comes to hatching which spawns its own set of problems.
In answer to your question if you see a blood ring on day 18 it will have happened early on and there will be a lot of clear space in the shell, by day 18 the egg will be pretty much taken up by the chick and you shouldnt be able to see any space apart from around the air cell.