Fertilized vs Unfertilized Eggs

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In our modern society, many of us have become removed from our food and there are lots of myths swirling around about eggs.  One of the most common (and unlikely!) fears surrounding eggs is that you could pull an egg from the fridge, crack it open, and out pops a fully formed chick.  So let’s take a look at that incredible, edible egg to sort out fact from fiction concerning fertilized eggs.

Quick chicken biology 101

Hens begin to lay eggs somewhere around the 16th-20th week of life.  Like human females, a hen is born with all the eggs she will ever have.  Thousands of tiny ova (the things destined to be egg yolks) lie within her ovaries.

When she reaches maturity, the ova is released into the oviduct where it will begin it’s journey, along the way it adds the egg white, shell membranes, and lastly the shell. The entire process of forming an egg will take about 24 hours to complete.

The fertilization happens immediately after the yolk is released. It goes from the ovary into a “holding tank” called the infundibulum for about 15 minutes.  If the yolk is to be fertilized, it needs to happen in this 15 minute window.  Hen’s bodies can store a rooster’s sperm for up to a week after mating, to be used during this window.

Roosters are not needed for egg laying

The process of producing eggs will happen with or without the presence of a male chicken.  The releasing of an ova is regulated not by mating, but by the hen’s pituitary gland. The pituitary gland is located by her eyes, just beneath the thin wall of her skull.  The skull is so thin that light can penetrate it, stimulating the gland below.  The pituitary gland needs 14-16 hours of daylight stored before it will signal to the ovaries to release an ova.  This is why chickens lay less in the short winter days and more in the spring/summer.   So to lay eggs, hens need light NOT a rooster.

Chick Development

If the hen has mated within the past week, as long as the rooster is healthy, it is a good bet the eggs she lays will be fertilized.  But what does that mean for you as the egg eater?  Honestly it means nothing.

Even if you have a rooster, as long as you are collecting eggs every day you will not crack open an egg to find a developing chick.  For a fertilized egg to begin developing, it needs to be kept in the right conditions (generally around 100 degrees with 60% humidity) for several hours.

Even after several hours, the development is so slight, you would likely not notice it with the naked eye.  It take about 3 days of incubating at proper conditions for visible veining to show.

Let’s say you have a rooster and you forget to collect eggs for three days, but they are just sitting out in a nest box with no hen sitting on them. They are not going to be developing, unless for some reason your air temperature has been a steady 100 degrees day and night with moderate humidity.

But what if you have a rooster and a hen that is trying to sit on eggs?  If you are collecting them out from under her daily, they are not going to have a chance to develop.

Putting the eggs in the refrigerator will completely stop all development.  So if you had a fertilized egg sitting under a hen for a week, and you collected it and put it in the fridge, all development would halt. It will not continue to develop, but if you crack it open, you will likely see some development.  It takes 21 days of incubation for a chick to fully develop and hatch.

How can I tell if an egg is fertilized?

Other than having the potential for developing into a chick, there is no difference.  There is no difference in taste or nutritional quality, and you can’t tell from the outside.  When you crack open any egg (fertile or not), in the yolk you will notice a small white dot called a blastodisc.  In a fertilized egg, the small white dot will have a small bulls eye type ring around it and is now called a blastoderm.

Normal egg signs that don’t indicate fertility

Red spots or blood spots – this does not indicate fertility, it is just a broken blood vessel.  This is safe to eat and the occasional blood spot is normal.  The very earliest chick development at day four looks like veining, not spots.
White stringy “things” – some people think this is an umbilical cord, it is not.  It is called a chalaza and is a rope like structure that helps keep the yolk suspended in the egg so it isn’t banging against the shell.  The chalaza is more prominent in farm fresh eggs.  As eggs age, the chalaza begin to dissolve so you are less likely to notice these in store bought eggs.  It is totally safe to eat.

Bottom Line

If you are buying commercial eggs from the grocery store, it is very highly unlikely you would ever find a partially developed or developing chick in your egg.  Commercial egg farms only keep female hens so there would be no chance of them getting fertilized.

Even if some rogue rooster were to sneak in, the eggs at US egg farms are collected everyday, washed, and refrigerated so development would not have a chance to start.

If you found a partially developed chick in a commercial egg farm egg that is very concerning.  The hens are not being properly checked on if a mature rooster is hiding among them.  It also means the eggs are not being collected & processed often enough.

If you are eating eggs from a local small farmer or from your own backyard hens, even if a rooster is present, as long as eggs are collected daily, you do not need to worry about cracking open an egg and having a chick pop out.


9 comments

  1. Robin says:

    Great information! I’m always a little taken back by the thought that roosters are needed for egg production. I read it in a forum just the other day.

    Nice to find you through the Homestead Blog Hop!

  2. Valkyrie says:

    Thanks for the info, you made it clear and very easy to understand. I hope one day to have some chicks of my own but was worried that I would have to get a rooster; you’ve put that concern to rest.

    I am however, now concerned about the health of the chickens I used to spend a few weeks in the summer caring for when I was little; there were a lot of broken blood vessels that we were told was because they had a rooster and the egg was fertilized. Hmm :S

  3. Munchkn14 says:

    I had this happen to me when I was high school and bakin a cake! It was so gross. Ever since I crack eggs in a separate container first!! After seeing this I have no idea how it could have happened! The eggs were from the grocery store! (But it was like 20 years ago…..)

    • Liz says:

      I have no idea how that could ever have happened either! I know some countries don’t refrigerate eggs and if it was a small store that used local farmers – maybe, even then that is super strange!

  4. Síbhean says:

    Just got a fetus in my egg an hour ago and I’m absolutely befuddled. I’ve always known the chances of this are in the hundredths or thousandths of a percent, so when I cracked the egg and saw a red splotch I thought nothing of it and poached it business as usual. Lo and behold, 10 minutes later I sat down, bit in, saw the other half of the fetus in the other half of the egg and immediately spit the half in my mouth out. Didn’t think to get a photo before my cat took advantage of my stupefaction and swiped it. I also don’t know how I missed it before I started eating. I guess ’cause it was in water it was floating under the yolk where I couldn’t see it.

    I’m still boggled. These eggs definitely aren’t local farmers market stuff. They came from either Kroger or Aldi (not sure which, but those are the stores around here in southern Ohio). Judging by photos of egg growth stages, this egg was sitting there growing for a good 5-8 days before it was packaged up and refrigerated to death. Definitely won’t be buying that brand ever again, even (or maybe especially) if they are on sale super cheap :/

    • Liz says:

      Wow! That is very disturbing. I would be very hesitant to buy that brand again, if they can’t figure out that they have a rooster among the hens by the time it has become mature enough to fertilize eggs they need some major overhauls! Sorry you had to experience that!

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