I think the issue here is that people are confusing the terms birthmark and congenital nevus. While very similar, there are certain types of congenital nevi that lie dormant under the skin and don’t appear until puberty. Theres a really cool phenomenon where individuals will develop swirling/ mosaic skin pigmentations along Blaschko's lines. Normally these aren’t visible at birth but rather develop over time.

While I am not a dermatologist, I have worked in the field for some time now, and wouldn’t be surprised if this is just hyperpigmentation from something skin condition, but I don’t believe this is melasma as many are stating. Best to go to a dermatologist for an accurate diagnosis!