Chick Embryo Development Day‑by‑Day

Egg candling is the process of shining a bright light through an egg to observe the interior contents without breaking the shell. It's essential for monitoring embryo development, identifying infertile eggs, detecting early death, and tracking the growth of the air cell. Candling helps improve hatch rates by allowing you to remove non-viable eggs before they spoil and potentially contaminate healthy ones.

The three most recommended candling days are Day 7, Day 14, and Day 18. On Day 7, you can clearly see blood vessels and confirm fertility. Day 14 allows you to check for continued development and spot any stopped embryos. Day 18 (lockdown) is the final check before the chick positions itself for hatching. Avoid excessive candling as it can disturb the developing embryo and cause temperature fluctuations.

You need a bright, focused light source in a dark room. Options include: a dedicated egg candler (available online or at farm supply stores), a small LED flashlight with a narrow beam, or even a smartphone flashlight. The key is to create a tight seal between the light and the eggshell so light passes through rather than around the egg. A dark room is essential for seeing details clearly.

A fertile egg will show a network of fine blood vessels radiating from a small dark spot (the embryo) by Day 5–7. An infertile egg appears uniformly bright and clear, with only the yolk shadow visible—no blood vessels, no dark embryo. If you see a blood ring (a red circle inside the egg), this indicates an embryo that started developing but died early (usually within the first 3 days).

At Day 7, a healthy embryo appears as a dark spider-like shape with a clearly visible network of branching blood vessels spreading outward. The embryo itself looks like a small dark dot surrounded by a web of red lines. The egg should have a warm, pinkish glow overall. If you gently rotate the egg, the embryo may shift slightly. This is one of the most reliable days for confirming viability.

When done properly and sparingly, candling does not harm the embryo. However, excessive or prolonged candling can cause the egg to overheat from the light source, and frequent handling increases the risk of contamination or accidental damage. Limit candling to 2–3 sessions during the 21-day incubation period, keep each session brief (under 30 seconds per egg), and always handle eggs with clean hands.

The air cell is a pocket of air located at the blunt end of the egg. It forms as the egg cools after being laid and the inner membrane separates from the shell. During incubation, the air cell gradually enlarges as water evaporates from the egg. By Day 18, it should occupy about 1/3 of the egg's interior. A properly sized air cell is critical—too small indicates excessive humidity, too large indicates insufficient humidity.

Signs of embryonic death include: a blood ring (early death, Days 1–5), collapsed or disintegrating blood vessels that appear dark and murky, no visible movement when the egg is gently rotated (after Day 8–10), a uniform dark but cloudy appearance without clear structures, and a foul odor if the egg has spoiled. The embryo may also appear as a shapeless dark mass rather than a defined form with visible blood vessels.

After Day 14–16, the growing chick fills most of the egg's interior, leaving very little space for light to pass through. The embryo's body, feathers, and organs are dense enough to block light almost entirely. At this stage, you'll primarily see the air cell at the blunt end as a bright area, while the rest of the egg appears dark. This is a positive sign indicating a large, healthy, near-term chick.

A quick final check on Day 18 is acceptable and recommended before lockdown. After Day 18, avoid opening the incubator—the chick is positioning itself for hatching, and disturbance can cause malpositioning. The high humidity needed during lockdown means opening the incubator causes a damaging drop in humidity. If you must candle during lockdown, be extremely brief and minimize incubator opening time.