Where do birthmarks come from?

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Where do birthmarks come from?

In: Biology
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The short answer is that there are many types of birthmarks with different causes. Some are related to unusual areas of skin pigment, some are due to unusual blood vessel structures, some are from differences in the layer of fat below the skin and many more.

This site has a lot of information, but in pretty technical terms. https://dermnetnz.org/topics/congenital-naevi/

There are 2 types of birthmarks: pigmented birthmarks and vascular birthmarks. Pigmented birthmarks are caused by excess skin pigment cells. Vascular birthmarks are caused by increased blood vessels. Birthmarks are a result of a localized imbalance in factors controlling the development and migration of skin cells.

Source: [Wikipedia](https://en.wikipedia.org/wiki/Birthmark)

I do know that for one congenital condition, neurofibromatosis, it can be common to have a coffee colored birth mark on your body. It could be anywhere, any shape. Not in all cases, but if it runs in the family, like it does in mine, you can let the doctor know that you have the birthmark and that it runs in your family. I had some genetic testing done, as well as my daughter, and found we are both genetic carriers. My grandmother, her twin, my uncle and my aunt had it severely. I don’t know what, if any, other congenital conditions may be linked to birthmarks.

When the skin cells develop and start multiplying to cover the fetus in the uterus, there can be a genetic change in one of the cells for whatever reason. Any daughter cells from that cell would have the same genetic change, and that results in a group of cells that are slightly different from all other skin cells. That results in a birthmark.

None of these really address the core of what you’re asking: why does nearly everyone have on, and why is it almost always just one?

I know exceptions exist; many people don’t have a birthmark, many people have multiple. But by and large, people have exactly one. Why? Describing what causes it doesn’t explain the phenomenon of each person having one unique birthmark.

Hi y’all,

This is what I like to call a ‘universal experience thread’. Almost everyone has a birthmark or knows someone who has. As a consequence of that ubiquity, threads like this tend to get a lot of anecdotal replies.

Here at ELI5 we try to maintain a focus on simplified explanations of complex concepts. Anything that isn’t an explanation as described in rule 3 can’t be a reply directly to the OP. That ensures that the sub reliably sees good explanations rise to prominence.

**In this thread in particular a lot of people want you to know about [this relevant podcast episode](https://podcasts.apple.com/us/podcast/stuff-you-should-know/id278981407#episodeGuid=4311c19a-53a3-11e8-bdec-2fb3b5e218e7) and that various old-wives-tales are nonsense.** ^(Our rules require links to include written explanations, so many were removed.)

Having a comment you spent time crafting removed is a bummer. We like to give a little warning, when we can, to try to save some people from that.

Keep in mind that replies to other comments don’t have that same standard applied to them.

[Here’s a link to the rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules), which have recently been rewritten to be more informative/clear.

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If you want **even more words** look at the reply below. Users love more words[,](https://youtu.be/vIpk8A8Ts3s?t=8) ^always.

Out of the massive amount of cells that are replicating while you grow, some accidentally make a minor mistake.

When cells make major mistakes, they got quality checking that will dispose of them. Minor mistakes, like if you get extra melanocytes in one spot or if a capillary keeps growing when it shouldve stopped, can make it through as long as theyre relatively normal for their cell type.

There are 2 general types: one is due to blood vessels, the other is due to melanin concentration. There is a recent episode of Stuff You Should Know podcast that talks about birthmarks. [Sysk podcast: birthmarks ](https://open.spotify.com/episode/4S30TxZNd1vyTc2pBDZn8h?si=CbkoJ9v_SKWjy71tsfb1uA)