Where do birthmarks come from?
In: Biology
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)
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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.
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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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As always, I am not the final authority on any of this. If you want my mod-action reviewed you can [send a modmail](https://www.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive). If you want to have a meta-conversation about the rules of the sub you can make a post in r/ideasforeli5 which is our home for that.
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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.
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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.
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