Pregnancy increases your body's demand for folic acid. When you have low folic acid levels in your body, you may develop magaloblastic anemia. With this type of anemia, RBCs are enlarged and deformed, resulting in tiredness and weight loss. Low levels of folate during pregnancy also increase the risk of neural tube defects in your baby.
While pregnant, you also increase your need for iron to 30mg a day. Lack of iron may lead to iron deficiency anemia, which may be associated with increased weight or size of the placenta. If you have low hemoglobin levels, you have a higher risk for delivering preterm or babies with low birth weight.
During pregnancy, you also retain fluid easily, which may produce high volumes of plasma, the fluid component of blood. This can dilute RBCs, which may lead to anemia.