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There are three main reasons why you might get anemia:
You lose too much blood.
Your body doesn't make enough red blood cells.
You have a disease or other problem that destroys red blood cells.
Losing too much blood
This is a common cause of anemia, especially for women who have heavy bleeding during their periods.
It can also happen with ulcers or other problems that cause bleeding inside the body.
Not making enough red blood cells
A balanced diet usually provides the vitamins and minerals your body needs to make red blood cells. You might get anemia if your food doesn't include enough iron, folic acid, vitamin B12, and vitamin C.
A pregnant woman needs to make extra red blood cells for the growing baby. So anemia during pregnancy is common.
Sometimes a long-term disease keeps your body from making enough red blood cells. Examples include kidney disease, arthritis, diabetes, and cancer.
Destroying too many red blood cells
Red blood cells last about 4 months. That's why your body needs to keep making new ones. But there are some problems that can destroy red blood cells sooner than that.
For example, diseases like sickle cell disease and thalassemia destroy red blood cells before their time. And some medical treatments, like chemotherapy, can destroy red blood cells.