How a Chicken Lays an Egg Inside of an Egg

Egg Inside Egg? The Science Behind a Counter-Peristalsis Contraction

When raising chickens for eggs, expect the unexpected. Though quite rare, it is well-known that occasionally a hen will lay an egg inside of an egg. The cause of this phenomenon is called a counter-peristalsis contraction and occurs while the hen is in the process of forming an egg in her oviduct.

How do chickens lay eggs normally? It works like this: A hen typically releases an oocyte (the ovum that becomes the yolk of an egg) from her left ovary into the oviduct every 18-26 hours. The oocyte travels slowly through the oviduct organ adding layers of the egg along the path to the chicken’s vent from which it will lay the egg.

How a Second Egg is Formed

A counter-peristalsis contraction is when a second oocyte is released by the ovary before the first egg has completely traveled through the oviduct and been laid. The release of a second oocyte into the oviduct system while a first oocyte is in the eggshell-gland portion of the oviduct (the eggshell gland is also called the uterus in a hen and is where the shell is deposited over the egg) causes a contraction. This counter-peristalsis contraction, resulting from the premature release of a second oocyte into the oviduct, causes the first egg in the eggshell gland to reverse its course and be pushed back up to the top of the oviduct. Consequently, the first egg (i.e. the previously released egg which was in the lower portion of the oviduct before reversing course) is typically added to the oocyte that was just released into the oviduct. The second oocyte then travels down the oviduct and has albumen and a shell deposited over it and first egg together. This creates a very large egg for your poor hen to lay. Ouch! When you crack open such an egg, there is normal yolk and whites as well as another fully formed, normal-sized egg inside.


A Mini Egg Inside an Egg (Regular Size)

Recently, a tiny, fully formed egg was found inside of a regular-sized egg in Britain. This particularly rare, miniscule egg inside an egg was also caused by a counter-peristalsis contraction. However, in this case, the oocyte that was released in the first egg (the one that reversed course in the oviduct) was tiny because the ovary had released an oocyte out of order. Usually, hens ovulate daily in order of size – laying the largest, most developed oocyte first. The hen’s ovary is simultaneously preparing smaller oocytes for release at a later time. Occasionally, a small, underdeveloped oocyte jumps the queue. In case of the British man who found the tiny egg inside a normal-sized egg – that is what happened.

Other Egg Inside An Egg Videos

You can learn more about the formation of eggs and phenomenon of having a chicken lay a fully formed egg inside of an egg in episode 030 of the Urban Chicken Podcast  LISTEN HERE.

Want to learn more amazing eggs facts? Backyard Poultry answers your hardest questions about raising laying hens, including: Do different chicken egg colors taste different? Why is my chicken laying soft eggsHow old do chickens need to be to lay eggs?

Originally published in 2014 and regularly vetted for accuracy.

  • Friend of son and his wife had a hen lay this type of “double” egg. The first egg had 2 yolks and the inside egg, which yielded one yoke. First I’d seen or hear of this.

  • I would have freaked out and drop it or freak out and told my mom.I would have cooked it or sold it .

  • I think its cool how eggs can come in different sizes and colors. It’s even cool that an egg can be inside another egg.

  • Just boiled eggs for deviled eggs this morning and when I went to cut one of them on half I noticed a tiny “second yolk” and was able to easily remove it from the cooked egg white. After a closer look I realized it was actually a tiny second COMPLETE egg, shell, white, yolk and all!!! It was approximately 1/4″ in diameter. So cool!

  • My cousin Darryl Pillay, was making us breakfast and cracked open a normal sized Egg, bought at the local supermarket and found another fully formed tiny egg inside with it’s egg shell. That was amazing.

  • I found a tiny fully formed egg with shell inside my normal size egg. I had hard-boiled them. I have pictures too.

  • Hello this morning I found a mini unshellled egg in my normal egg

  • Dorothy M.

    One of my guinea hens once laid a perfect egg WITHOUT A SHELL at all. Since the membranes of their eggs are thicker than chicken eggs, it was still completely intact when I found it, and I got a couple of good pictures of it.

  • Opens our egg within the egg this morning. Videotaped the opening and the larger egg had a double yolk. This is our first year with chickens so it was really cool. Can share the video if you want it for your site

  • So my big question is, what would happen to the eggs if both chicks tried to develop? Would one be harmed in growing or possibly die? Or would you have two perfectly normal chicks?

  • I had a fully formed egg inside another egg yesterday (April 14, 2018). I should have taken a picture. I’very also experienced the phenomenon of a hen crowing – at that time I didn’t have any roosters. I’m half wondering what’s next!

  • We eat only duck eggs due to chicken eggs allergy.Most of the eggs have double yolks, normal size.Today I just found a fully formed egg, with the skin, inside another egg that already had an yolk.I was wondering if ,after removing the smaller egg,the rest is safe to eat.It doesn’t have a smell,but I am still scared. We buy the eggs from the farm and are almost as big as my palm.

  • Thank you for explaining the mini egg I found in my store bought chicken egg this week. (I’ve saved it in the refrigerator.)


Leave a Reply

Credit Card Identification Number

This number is recorded as an additional security precaution.


American Express

4 digit, non-embossed number printed above your account number on the front of your card.


3-digit, non-embossed number printed on the signature panel on the of the card immediately following the card account number.


3-digit, non-embossed number printed on the signature panel on the back of the card.

Enter Your Log In Credentials
This setting should only be used on your home or work computer.


Send this to a friend

Shared with you:

How a Chicken Lays an Egg Inside of an Egg