# Fingerprints and amniotic sacs

**URL:** <https://boards.straightdope.com/t/fingerprints-and-amniotic-sacs/650644>\
**Category:** Factual Questions\
**Created:** [February 18, 2013, 12:08am UTC](https://boards.straightdope.com/t/fingerprints-and-amniotic-sacs/650644 "2013-02-18T00:08:20Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\
**Post date:** [February 18, 2013, 12:08am UTC](https://boards.straightdope.com/t/fingerprints-and-amniotic-sacs/650644/1 "2013-02-18T00:08:20Z")

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I was recently reading about telling twins apart, which linked to [this](http://genetics.thetech.org/ask/ask68) article, which includes the paragraph:

> [@The Tech website](#):
>
> While you were growing inside of your mother, you touched the amniotic sac. When you touched it during weeks 6-13, the patterns of your fingerprints were changed. This is why identical twins have different fingerprints.

Searching around, this idea seems to be somewhat popular, but there doesn’t seem to be a mechanism mentioned, and I don’t find it believable. How would this theory be of fingerprint occurrence have been invented, tested, and validated? What’s the process here that I’m unaware of?

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**Author:** ![rhysf](https://avatars.discourse-cdn.com/v4/letter/r/f14d63/32.png) [@rhysf](https://boards.straightdope.com/u/rhysf)\
**Post date:** [February 18, 2013, 12:50am UTC](https://boards.straightdope.com/t/fingerprints-and-amniotic-sacs/650644/2 "2013-02-18T00:50:39Z")

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My reading of the scientific literature doesn’t mention that theory at all. The consensus seems to be that the processes governing fingerprint formation are unknown but genetics makes some contribution.

The timing is also wrong. From [here](http://www.tbiomed.com/content/8/1/24) and [here](http://www.sciencedirect.com/science/article/pii/S0022519304006198), fingerprints form between 10 and 19 weeks. At six weeks, the fingers haven’t even separated.