The phrase "mg iron" refers to a unit of measurement used to indicate the amount of iron in something. The "mg" stands for milligrams, which is a very small amount. So, "mg iron" means a small quantity of iron. Full definition
Taking a 30 to 50 mg iron supplement during the crucial second half of your pregnancy will ensure that you're getting what you and your baby need. (whattoexpect.com)
A Cochrane review showed that daily supplementation with 9 — 90 mg iron reduced the risk of anemia in pregnant women at term by 70 % and of iron deficiency at term by 57 % [55]. (ods.od.nih.gov)