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Fertilized and Unfertilized Eggs: What Is the Difference?

Last Updated: 25.02.24


If you want to raise hens and you need equipment, one of our recent posts is packed with suggestions. This short article can help you distinguish between fertilized and unfertilized eggs. It also explains the egg production process and it presents a couple of myths about fertilized eggs.


What is the difference between fertilized and unfertilized eggs?

Hens do not need roosters to lay eggs. When a hen reaches maturity, it starts laying eggs, whether or not it mates with a rooster. In industrial egg production farms, because hens do not live in the same space as roosters, hens cannot produce fertilized eggs. Therefore, the eggs that you purchase from the store are not fertilized.

Even though an egg is fertilized, once it is placed in the freezer, the development of the embryo is stopped. So, there is no danger in eating eggs that have been fertilized. Because all eggs sold for consumption the US have to be refrigerated, the chances of you ending up with a fertilized egg on your plate are minimal.

One fun fact that you should know is that a hen can lay fertilized eggs from two days to three weeks after mating. 


How are eggs formed inside the hen?

The formation of the egg starts with the yolk. At this stage, the yolk is called an oocyte and it is produced inside the ovary of the hen during the ovulation process. Next, the oocyte is released inside the oviduct, a tube that is part of the reproductive system of the hen. If the hen has mated with a rooster, the yolk is now fertilized.

Then, the yolk travels down the oviduct into the magnus and it is now covered in the vitelline membrane, layers of albumin, this is the scientific name for the egg white, and fibers. As the egg goes down the tube, it is now covered in structural fibers that anchor the yolk into the egg white. 

Finally, the formed egg is covered in a shell that is made of calcite, a form of calcium carbonate. The entire process takes no more than 24 to 26 hours. 



How can you tell if an egg is fertilized?

When cracking open an egg that has been fertilized, you will be able to see a small white dot that has a bull’s eye type ring surrounding it. This is called a blastoderm. Fertilized eggs have the same taste as unfertilized ones and they have the same nutritional quality.

If you see blood spots or red spots in the egg, this is not a sign of fertility. This is just the result of a broken blood vessel in the hen’s reproductive system. In most cases, broken vessels are a result of a vitamin A deficiency. What is more, the specialists argue that these eggs are also safe for consumption.

Similarly, if you spot white string-like things inside the egg, this is also not a sign of the egg being fertilized. The white string is called a chalaza and it is the rope-like structure that keeps the yolk suspended inside the egg so that it does not crash into the shell.

Chalaza is oftentimes prominent in fresh eggs. As the eggs age, the chalaza is dissolved and it is difficult to spot. If you happen to find an egg that has a chalaza that is visible, you can still eat it because it is safe.


Are there tests one can use without cracking the eggs?

If you want to know whether an egg is fertilized or not, but you don’t want to crack it open, there is one thing that you can try.

This test is very old and is known as the candling of the egg. The test requires a person to hold the egg up to a lit candle or a very bright flashlight. If the egg looks cloudy, it is, most likely, fertilized. Yet, it should be pointed out that, even if you try this test, chances are that you won’t be able to see anything before the 3rd day of fertilization. 

Because eggs are collected and refrigerated daily, your chances of spotting something are close to none. Once again, even if an egg is fertilized, it is still safe to eat. Thus, you should not throw it away.


Fun facts about fertilized eggs

According to researchers, hens can tell which eggs are healthy and, thus, likely to hatch correctly. So, before it starts brooding, the hen will remove the eggs that it considers unviable from the nest.

The same is true with the chicks. If a chick appears to be sick, the hen will devote less attention to it. Instead, she will take better care of the stronger ones that are more likely to survive.

An egg contains all the nutrients that a chick needs to thrive over a couple of weeks. One single egg contains 5 grams of fat and vitamins as well as 7 grams of protein. As the chick grows and develops inside the egg, it slowly absorbs these substances. After 21 to 23 days, the chick is ready to hatch.

Even more interestingly, in the final hours of the hatching process, the remaining yolk is absorbed into the abdomen of the chick. This way, the chick is well fed for at least 24 to 36 hours before it actually hatches.

The temperature of a freshly laid egg is of no less than 105 Fahrenheit degrees. However, the egg soon cools off. This is the reason why broody chickens maintain the eggs that they are sitting on at a temperature of around 99.5 to 102 Fahrenheit degrees. 

Even more so, to make sure that the chicks inside the eggs develop as expected, the hens turn the eggs regularly.  

While many Americans fear that they will crack an egg and find an embryo, in East Asia, you can purchase eggs that have formed chicks inside them. These eggs are called balut and they are considered a delicacy. So, after all, it seems like there is some truth to the saying that a man’s trash is another man’s treasure!



Myths about fertilized eggs

Some people argue that fertilized eggs have a distinct taste that differentiates them from unfertilized ones. However, there is no scientific basis for this claim. Additionally, some say that a fertilized egg is more nutritionally valuable than an unfertilized one. Once again, this is only a myth that is not backed by scientific research.

Another fun myth that many believe is that the shape of the egg indicates the sex of the chick. According to some folks, an egg that has a pointy shape will hatch into a rooster, while a rounder egg will hatch into a hen. 


How many eggs can a chicken lay?

A hen can lay one egg a day but it can also have days when it does not lay any eggs. Because of the laying schedule that each hen has, normally, an egg is formed shortly after the hen has laid the previous one.

The reproductive system of a hen is greatly influenced by the presence of light. Therefore, if the hen has laid an egg late in the day, its body will not begin the formation of a new egg until the next day. This is the reason why, in egg farms, in an attempt to efficientize the process, hens are exposed to light at all times. 

What is more, some hens are specially bred for egg production. On average, a hen of this type can lay up to 276 eggs per year. The productivity of hens is influenced by factors such as breed, environment, feed and life span. Backyard chickens lay fewer eggs that those raised in factories. 


Impact of egg-laying on the health of the hens

The rate at which hens lay eggs nowadays, especially in factories, is unnatural when compared to the number of eggs laid by wild hens. As studies have shown, wild hens lay only 10 and, in rare cases, 15 eggs each year. This happens because they only lay during the breeding season in spring.

Because of the exploitation of hens in factories, hens have become, as some critics argue, prisoners. The overproduction of eggs also causes numerous diseases of the reproductive tract of hens such as osteoporosis or skeletal paralysis.

Up to 95% of all the egg-laying chickens in America spend their entire lives trapped in cages and they don’t have the needed room to spread their wings. Once they stop laying the expected number of eggs, the hens are killed. Most hens do not live more than two years in egg production factories. After two years, most of them have numerous broken bones and they are in excruciating pain.


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