The way you phrased that makes me wonder if you are counting the days right. Lockdown normally starts after 18 full days of development. An egg doesn’t have a full day of development 2 seconds or 2 hours after you put it in the incubator. It takes 24 hours of development for the egg to have a full day’s worth of development. If you started the eggs on Tuesday August 13, 18 days of development are complete on Saturday August 31 about the same time of day you started them. In theory they should hatch Tuesday September 3. An easy way to check yourself is that the day of the week you started them is the day of the week they should hatch, in this case Tuesday.
I say in theory because eggs often don’t hatch exactly 21 days after you start them. There are a lot of things that can affect exactly when an egg hatches; heredity, humidity, how and how long the egg was stored before incubation, and just basic differences in individual eggs. Average incubating temperature is real important. If your incubator is running a bit warm, they can be early. If the incubator is running a little cool, they can be late. I’ve had eggs in an incubator and under a broody hen hatch 2 full days early. Some people have reported them hatching much later. So don’t get too obsessed by that 21 day thing. It’s a target to aim for but many of us regularly miss it a bit.
Usually my hatches are pretty much over within 24 hours of the first one hatching. But I have had some drag on for over two full days. In my last incubator hatch, I had one chick hatch a full day before any other egg pipped, then the other 17 came out basically overnight.
Hatching is not an instantaneous process either. The chick has to position itself for hatch, internal pip, external pip, and finally zip. During this time it is absorbing the yolk, drying up blood vessels, learning to breathe air instead of living in a liquid environment, doing something with that gunk it has been living in so it dries nice and fluffy instead of all gunked down, and who knows what else. Some chicks do a lot of this before external pip. These usually hatch a few hours after you see the pip. Some do a lot between pip and zip. These drive us crazy waiting on them to finish up.
The reason I’m writing all this is that I think this might be your first incubator hatch. It’s an exciting time but it can be really stressful. You’ll be wondering what you did wrong, why aren’t they hatching (or why are they early), and what should you do. Most of the time it works out well. I wish you luck.
