high-risk

Well as you might expect, for women a key reason is heavy menstruation. Other factors are also at play including a diet poor in iron, poor iron absorption from the gut, bleeding from the gut (which may not be obvious) and multiple blood donations. Other high risk groups are:
[1] adolescent girls, who need iron as they are growing, who also have to replace iron from menstruation, but like many women do not have adequate iron in their diet;
[2] women with heavy or prolonged periods
[3] vegetarians, who do not take haem iron and may also be lacking B vitamins
[4] highly active men and women need extra iron for energy production in their muscles
[5] pregnant women need extra iron as the baby grows and also as the blood volume expands

high-risk

http://www.nhlbi.nih.gov/health/health-topics/topics/ida/signs