The first time I candled eggs I used my phone flashlight in a closet and squinted at twelve glowing eggs, seeing nothing but light and yolk. I was on day 4. Nothing to see yet, and I chilled the clutch for nothing. Candling is simple, but the timing is everything: candle too early and you see nothing, candle too often and you disturb the chicks, candle too late and you cannot catch the bad eggs in time. Three days matter. Here is what each one shows you.
What you need: less than you think
A bright LED flashlight works. A dedicated egg candler works better. Your phone flashlight works in a pinch, though the beam is wide. You need a dark room, clean dry hands, and ideally a notebook to track which eggs looked like what. Practice on a store-bought egg first so you know what a clear, unfertilized egg looks like. That baseline makes the developing ones obvious.
Technique: hold the egg gently, large end up, light against the shell. Rotate slowly. Keep it quick. Then put it back. The whole point is information, and information does not require a photoshoot.
Day 7: the fertility check
Day 7 is the first day worth your time. Earlier than this, the embryo is too small to read reliably, and you risk tossing viable eggs or, worse, chilling the whole batch for no information.
A fertile egg on day 7 shows a network of blood vessels spreading outward like spider webs, with a small dark spot, the embryo, near the center. That veining is unmistakable once you have seen it. An infertile egg is clear: light passes through, you see a faint yolk shadow, and nothing else.
If an egg is completely clear on day 7, it is almost certainly not developing. Some keepers leave questionable eggs one more week to be sure. I do not. A clear egg on day 7 that is still clear on day 10 was never going to hatch, and every day it sits in the incubator it risks rotting and contaminating the healthy eggs around it.
Day 14: the progress check
By day 14 the chick fills much more of the egg. You will see a larger, darker mass, more defined veins, and sometimes actual movement, a twitch or shift as the embryo repositions. The air cell at the large end should be clearly defined and growing.
This is also the check where problems show up. A blood ring, a red ring floating in the egg with no veining and no dark mass, means development stopped early. It happens even with perfect incubator settings and healthy parent flocks. It is not something you did wrong. Remove the egg and move on.
Dark-shelled breeds, Cochins and Marans keepers know this well, are hard to read early. If you are squinting on day 14 with a dark egg, look for depth and solid shadowing rather than fine detail. Shadowing is a good sign even when you cannot see veins.
Day 18: the final check before lockdown
Day 18 is the last look. By now the egg is mostly dark. The chick fills nearly all available space. The air cell at the large end is prominent. If you see that full, dark egg with a clear air cell, everything is on track.
After this check, lockdown begins: stop turning, raise humidity, close the lid, and keep your hands off. The chicks are positioning themselves for pipping, and opening the incubator drops the humidity they need to rotate inside the shell. The most common lockdown mistake I see is one last "just checking" candle on day 19 or 20. Resist it. Trust the process.
The mistakes that actually hurt hatches
Candling on days 1 through 5. There is nothing to see, and you chill the eggs for nothing. Wait for day 7.
Marathon sessions. Eggs cool fast out of a 99.5 degree incubator. Candle a few, return them, candle a few more. Ten minutes out of the incubator is the ceiling, not the target.
Dirty hands. Wash and dry before you touch incubated eggs. Oils and bacteria on the shell are a real contamination route.
Leaving bad eggs "just in case." A confirmed blood ring or a clear egg at day 14 is not going to surprise you. Rotting eggs can explode, and the word I want you to sit with is "explode," because that is what happens, coating your good eggs in bacteria.
Planning your hatch around these dates? Use the Egg Incubation Calculator to get your set date, candle dates, lockdown, and hatch window for chickens and 14 other poultry species.
The short answer
Candle on day 7 to confirm fertility (look for spider-web veins and a dark embryo spot), day 14 to check progress (darker mass, movement, growing air cell), and day 18 for the final check before lockdown (egg mostly dark, prominent air cell). Clear eggs and blood rings come out; everything else stays in. After day 18, hands off. Three checks, quick sessions, dark room, and you will know exactly which eggs are on track.
Related: Lockdown Day 18: 7 Mistakes That Kill Hatches and Duck Eggs vs Chicken Eggs in the Incubator.
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