This study was to evaluate the prevalence of iron deficiency anemia and the associated factors of anemia during pregnancy in the lower north of Thailand. A prospective study was conducted in pregnant women who attended the antenatal care clinic at Naresuan University hospital, Phitsanulok, from May 2008 - May 2010. All subjects were from the lower northern region of Thailand. A complete blood count and serum ferritin was performed in every case. Anemia during pregnancy was defined according to World Health Organization criterion as the hemoglobin less than 11 g/dL. Iron deficiency anemia was the anemia with a serum ferritin of less than 15 mg/L. Four hundred and one pregnant women were recruited for the study. Teenage pregnancy was 27%. The gestational age at the first visit ranged from 5 to 38 weeks and the iron supplement was found in 30% of cases. Anemia was found in 70 cases (17.5%). Twenty-four cases (6.0% of 401) had iron deficiency anemia but 61 cases were at the stage of iron deficient erythropoiesis without anemia. Associated factor for iron deficiency anemia was gestational age. This study confirmed a high prevalence of anemia and iron deficiency anemia. High proportion of pregnancies with iron deficient erythropoiesis was warranted.