Types of Anemia During Pregnancy

Several types of anemia can develop during pregnancy. These include:

• Iron-deficiency anemia
• Folate-deficiency anemia
• Vitamin B12 deficiency

Here's why these types of anemia may develop:
Iron-deficiency anemia. This type of anemia occurs when the body doesn't have enough iron to produce adequate amounts of hemoglobin. That's a protein in red blood cells. It carries oxygen from the lungs to the rest of the body.
In iron-deficiency anemia, the blood cannot carry enough oxygen to tissues throughout the body.
Iron deficiency is the most common cause of anemia in pregnancy.
Folate-deficiency anemia. Folate, also called folic acid, is a type of B vitamin. The body needs folate to produce new cells, including healthy red blood cells.   
During pregnancy, women need extra folate. But sometimes they don't get enough from their diet. When that happens, the body can't make enough normal red blood cells to transport oxygen to tissues throughout the body.
Folate deficiency can directly contribute to certain types of birth defects.
Vitamin B12 deficiency. The body needs vitamin B12 to form healthy red blood cells. When a pregnant woman doesn't get enough vitamin B12 from her diet, her body can't produce enough healthy red blood cells. Women who don't eat meat, poultry, dairy products, and eggs have a greater risk of developing vitamin B12 deficiency, which may contribute to birth defects. 
Blood loss during and after delivery can also cause anemia.

see also Risk Factors for Anemia in Pregnancy