Foetal blood vessel. Coloured scanning electron micrograph (SEM) of a fracture through a foetal blood vessel. Foetal erythrocytes are larger and have more variation in shape and size than those of adults. They are largest early in gestation. Although nucleated (brown nuclei) red blood cells are rarely found circulating in older children, they are commonly seen in the blood of newborns. Magnification: x5000 when printed 10 centimetres high.