Iron deficiency is the most common nutritional deficiency in the U.S. currently known. It is most common among children and women of child-bearing age, and its consequences are especially serious for fetuses and young children.There are 3 stages of iron deficiency: iron depletion, erythropoiesis, and anemia.

A person suffering from iron depletion has a reduced amount of storage iron, but his/her levels of functional iron are unaffected. His/Her health therefore is not directly at risk. However, should s/he undergo a stress that taxed his/her hemoglobin supplies, the deficiency would become more serious.

When erythropoiesis sets in, storage iron has been depleted and transport iron reduced further. The amount of iron absorbed in this stage does not meet tissue needs for growth, or even function, and cannot replace loss. Erythropoiesis limits the production of red blood cells.

Anemia affects multiple organ systems, and as a condition results in the underproduction of iron-containing functional compounds like Hb.

If a pregnant woman has an iron deficiency in her first or second trimester, there is a significant increase in risk for preterm delivery and low birth weight. Children aged less than 24 months (especially those between the ages of 9 and 18 months) have the highest risk of an iron deficiency, and being fed cow's milk or goat's milk before the age of 1 year increases this risk. The consequences for children with iron deficiencies are developmental delays and behavioral disturbances in the fields of motor activity, social interaction, and attention to tasks. An iron deficiency also contributes to the likelihood of lead poisoning, as the gastrointestinal system is more eager to absorb heavy metals. Factors that increase the risk of an iron deficiency for women post-puberty are heavy menstrual blood loss, the use of an IUD (intrauterine device) for birth control, and high parity. The use of oral contraceptives is associated with a decreased risk for iron deficiency.

My source for this writeup was, and I recommend it to anyone interested in this topic with a better understanding of medical terminology than me (which is a good chunk of the population I'm sure).