So funny, I've just been wondering about this and looking into it a bit. I saw something in which researchers said I think in around 30% of cases where someone had a birthmark and could remember details of previous lives, they found injuries matching the birthmarks. I don't know how many people were involved in the study though.
My son has a birthmark on his leg. I wonder whether it could reflect a severe burn in a previous life. My mum's a nurse and she also thinks it resembles a burn in form, though it is not a burn and is definitely a birth-mark.