There is lots of random reasons all over the internet for birthmarks and their placement. Do any of your children have birthmarks the same as a relative or do any of your children have a birthmark where you think you know why they got it in that particular place?
There are numerous people born with birthmarks in the shape of countries, hearts etc. I find it fascinating.
All 3 of my daughters have differing birthmarks on different parts of their bodies and none match to me or their dad.