A blood spot inside an egg can occur at various points in a hen's reproductive system as a result of a blood vessel rupturing. It can be the result of a genetic predisposition, a vitamin A deficiency, or a random event. There is no correlation between blood spots and fertile eggs. The misconception may have come about due to the appearance of incubated, fertile eggs developing veins at or around day four into incubation. Veining looks nothing like a blood spot, however.