The liver is the primary blood-forming organ until the sixth gestational month: then the bone marrow becomes the primary production site. At birth nearly all the bones are actively involved in cell production. By age four, hematopoiesis decreases in the shaft of the long bones and fat cells begin to be visible. At age 18 and throughout adult life the active cell producing sites are:

• skull
• scapula
• sternum
• ribs
• vertabrae
• pelvis

The long bones, tibia and femur, are active bone marrow sites from birth into the second decade.