A few seasoned chicken pros told me that if you want to introduce eggs that are a week away from hatching then you need a broody that has already been sitting for a couple of weeks at least. I learned the hard way in that I put 18 day incubated eggs under a hen that had only been broody for a week (it was a bit of a desperate move as the "intended" broody had rejected the eggs and launched them from the nest. The babies hatched on time on day 20 but mom squashed them. I'm told she wasn't ready and wanted to sit heavier as her instinct was to sit another 2 weeks. Makes sense to me actually, but this was my first hatch.I believe that two weeks is the minimum that is recommended. So, it sounds right.