An abnormally low level of red blood cells in the blood stream. Sufficiency of red blood cells is measured as the quantity of hemoglobin per milliliter of blood or the volume percentage of red blood cells in a milliliter of blood. There are several causes of anemia such as reduced production of red blood cells or increased blood loss. The most common cause is iron deficiency. Both pregnant women and children are at risk for a low level of red blood cells. The frequency of this problem is the reason that the red blood cell level should be measured as a part of routine care both in children and pregnant women.