Because plasma volume increases more than red cell volume, the hematocrit during pregnancy can be expected to fall to as low as 35% by the beginning of the third trimester. Generally, a hemoglobin value of less than 11 g per deciliter defines anemia during pregnancy.

Pure macrocytic anemia in pregnancy, with the presence of hypersegmented neutrophils, usually represents folate deficiency. Pernicious anemia is extremely rare during pregnancy. Anemia secondary to folate deficiency is frequently combined with iron deficiency and requires oral iron in addition to 1 mg of folic acid daily.

In nonmacrocytic anemia, a low serum ferritin concentration is diagnostic of deficient iron stores. Reticulocytosis can be demonstrated within 2 weeks of oral iron supplementation. Parenteral iron administration is rarely necessary.

Hemoglobinopathy or thalassemia should be suspected in patients who have a previous history of anemia, African or Mediterranean ancestry, and a normal serum ferritin concentration. Hemoglobin electrophoresis is necessary to establish the diagnosis.

The most frequently encountered hemoglobinopathies involve abnormal hemoglobins S and C and β-thalassemia, either isolated or in combination. Sickle cell trait (HbAS) is common but comparatively benign. Conversely, sickle cell disease (HbSS) and hemoglobin SC disease (HbSC) are frequently complicated by preeclampsia, infection, painful crises, and increased perinatal loss.