Menstrual blood loss is the primary cause. Women with heavier periods are at significantly higher risk. Dietary iron comes in two forms: haem iron from meat and fish, which is highly absorbable, and non-haem iron from plant sources including spinach, lentils, and sesame, which requires vitamin C to improve absorption.
Testing serum ferritin alongside haemoglobin gives a more complete picture of iron status. Ferritin reflects stored iron and can be low even when haemoglobin is normal. Supplementation alongside dietary improvement is usually warranted when ferritin is below 30 micrograms per litre.