These fingerprints start forming in the 10th week after conception when some cells in the middle layer of the skin, called the basal layer, start growing faster than the cells in the inner layer of dermis or outer layer of epidermis. The extra cells cause the skin to buckle and fold into ridges.
Source: Charlotte Lozier Institute
Part 1:
https://www.datebytype.com/blogs/view/120/Why-are-epigenetics-important-to-rh-negatives