Fertile Eggs: How to Spot Them

By Elena Johnson CVC AC

All eggs may not be fertile even if the hen has mated with a rooster. It takes 7-10 days for the sperm to reach the oviduct where the eggs are made. However, her eggs will remain fertile, even if she doesn’t mate again for around 2-3 weeks after mating. Fertility will begin to drop after 2 weeks. Not all roosters have fertile sperm.

There is no way to tell if an egg is fertile to put in an incubator without breaking it. So, if you really want to know if you have fertile eggs, you will need to sacrifice a few to crack open and check for fertility.

When a hen mates, the rooster’s sperm penetrates the egg and the blastodisc, a tiny white spot which carries the chick’s DNA, becomes fertilized. It’s then called the blastoderm or germinal disc. The blastoderm is what becomes the chick, if the egg is incubated at the right temperature and humidity levels.

If you crack an egg into a dish and it’s fertile, you will see a distinctive Bullseye shape. The bullseye is the first cluster of cells which IF incubated will become the chick. If the egg is NOT incubated, nothing happens, the potential chick is there but at this point it is simply a cluster of cells and not a chick. If it is incubated, the cells will start to divide and after even 24 hours you’ll be able to see definite growth in the bullseye. This development is very obvious in an egg that’s been cracked open but won’t be clearly visible in candling until about Day 5 of incubation. This is why it is so important if you don’t want to hatch chicks that you remove the eggs from the coop daily. 

When an egg is laid and cool, the division of the cells ceases. After the egg is laid, cooling the egg does not kill the embryo. It may resume its development after several days of rest if it is again heated by the hen or in an incubator. This is how hens hatch their eggs all about the same time. They will lay a clutch of 8-10 eggs before sitting on them for 21 days and the begin to hatch.

Fertility is generally lower in the winter and starts to increase as the days become longer and warmer. Peak season is between Easter and early summer and begins to fall as the darker, cooler days of autumn and winter approach.

It is completely safe to eat fertilized eggs. Unless it is incubated, that cluster of cells will not develop and despite what you may read on the internet, you are Not eating a baby chick. The white stringy stiff in the egg is not a chick, it is called the Chalaza. There’s one at each end of the yolk and it holds the yolk in the right position in the egg. In very fresh eggs the chalaza will look very thick, as the egg ages, it becomes thinner.