Blood loss is the most common reason for anemia

During pregnancy there is increased need for red blood cells

The three main reasons for developing anemia are blood loss, decreased production of red blood cells in bone marrow or increased destruction of erythrocytes. These factors, together or separately can cause anemia.

Blood loss

Blood loss is the most common reason for anemia and mainly iron-deficiency anemia. The blood loss can be transient or chronic. It could be caused from heavy menstrual period; bleeding from the digestive system or urinary tract; surgery; trauma or cancer. When the loss of erythrocytes through bleeding can not be compensated by the bone marrow, anemia develops.

Decreased production of red blood cells (erythrocytes)

The decreased production of erythrocytes is often due to inadequate intake of iron, folic acid, vitamin B12 or the reduced absorption of them in the digestive tract.
In some cases the body’s decreased ability to produce adequate amounts of red blood cells is a result from chronic illnesses like kidney disease and cancer.
Infections, drugs and X-rays can seriously damage the bone marrow, stopping his ability to be able to produce fast enough new red blood cells which to replace the braking erythrocytes.

During pregnancy there is increased need for red blood cells

With this increased need, if the mother’s body is unable to produce enough red blood cells for her and the embryo, it will lead to anemia.

Increased destruction of erythrocytes

Increased destruction of erythrocytes is usually observed with hereditary diseases, such as sickle cell anemia, thalassemia, or lack of certain erythrocyte enzymes. In these diseases there are abnormalities in the structure of red blood cells that lead to their rapid destruction.
In hemolytic anemia, the immune system attacks red blood cells, and it destroys them faster than the body can produce them.