A 30-year-old G1P0 woman complains of nausea and vomiting for the first 3 months of her pregnancy. She is noted to have a hemoglobin level of 9.0 g/dl and a mean corpuscular volume of 110 fL (normal 90-105 fL). Which of the following is the most likely etiology of the anemia

This is a macrocytic anemia because the mean corpuscular volume is above normal. Macrocytic anemias include folate deficiency and vitamin B12 deficiency; however, folate deficiency is more commonly seen in pregnancy than vitamin B12 deficiency. Iron deficiency is a microcytic anemia (MCV below normal), and it is the most common cause of anemia in pregnancy. Physiologic anemia of pregnancy is a result of the physiologic hemodilution that occurs in the vasculature. There is a dispropoionate increase in plasma volume over the increased RBC volume, and this "diluted state" also gives the appearance of a fall in the laboratory values of hemoglobin and hematocrit.