Eggs, period

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Eggs, period

A conversational strategy used by many vegans is to attempt to disgust the person who is eating animal products by describing what it actually is. It is a fair point that one should be aware of what one eats. I honestly think it’s quite scary how oblivious people generally seem to be about their food. This ignorance is usually exposed when discussing what vegans eat and do not eat with non-vegans. There are several different opinions on which strategy works best and one has to reflect on which strategy works in every given social circumstance. But, please stick to what is true. The egg of a hen is not menstruation or ‘a period’.


Both chickens and humans are in the group of amniotes, which is defined by a membrane during the development of the embryo. But then we are separated into synapsida – containing mammals, and diapsida – containing lizards, snakes and birds. These groups are defined by a certain opening in the skull. The most recent common ancestor between the wild ancestral chicken and humans dates back approximately 320 million years ago*. In other words, we are quite different from birds.


The ovary of the bird usually contains several egg cells that have started to mature. The maturation includes the formation of the yolk and growth of the follicle. When estrogen concentration goes up, proteins from the liver are deposited into the yolk to give it its yellow color. The egg will ovulate and fully mature which includes the formation of the egg white, and shell, but also potential fertilization. The eggs are typically laid until the clutch has been filled, but the interval and timing of egg laying varies between different bird species. A chicken will produce and lay an egg even if it is not fertilized*.

Birds do not have external reproductive organs, instead they have a cloaca from which they expel both eggs and excrement. Fertilization occurs when cloacas are joined and sperm is transferred into the female. However, birds are able to save sperm in an internal ‘pouch’, from which the female are able to fertilize eggs for a long period of time after the mating.


In the human ovary, an egg cell is released every ovulation. The ovulation occurs when luteinizing hormone and follicle-stimulating hormone peaks. When the egg cell starts to travel through the fallopian tube, the egg has a potential to be fertilized. During this time, the lining of the uterus starts to thicken to form an endometrium. The fertilized egg will eventually be completely embedded in the endometrium once it has reach the uterus. The endometrium supplies the embryo with oxygen and nutrients, and will later be the place where the placenta is developed*. If the egg is not fertilized, humans and a few other animals release the endometrium with the unfertilized egg through the vagina – this is called menstruation or ‘a period’. Many other mammals do not release the endometrium as humans do, it’s reabsorbed into the body. The latter is typically not referred to as menstruation, but if so, it’s called covert menstruation.


As people seem to be inadequately educated I guess it might sometimes come a legitimate opportunity to inform people what they are eating. But then it’s important to be correct. We all know for example that meat is muscle tissue of animals – that is correct. But, referring to eggs as menstruation is not. Birds have a different reproduction cycle than animals that have menstruation. The similarity is that both the laying of eggs and menstruation is a way of the body to get rid of an unfertilized egg, but in birds the egg is laid regardless if it is fertilised or not, and humans do not lay eggs at all. Additionally, menstruation is specifically defined by the discharge of the endometrium, something that just doesn’t happen in birds.

Apart from being factually wrong, speaking about menstruation with the intent of provoking disgust is problematic. In many parts of human society and history menstruation has been subject to taboo in various ways. Using arguments that rely on evoking disgust about female bodily functions might contribute to misconceptions and unreasonable antipathy. Menstruation is a completely normal function of the human body and shouldn’t be taboo in any way.

This article was originally posted on, and is reposted here with the permission of the author.

Simon Stenberg is a PhD student in molecular genetics. In his excellent blog Vegan Biologist he explores prevalent misconceptions and myths that run rife in the vegan community. By educating vegans on the scientific facts that underpin our arguments Simon is contributing to a stronger evidence based movement. Connect with him on Facebook to help fight irrationality.

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