Megaloblastic anemia is a type of macrocytic anemia. An anemia is a red blood cell defect that can lead to an undersupply of oxygen. Megaloblastic anemia results from inhibition of DNA synthesis during red blood cell production. When DNA synthesis is impaired, the cell cycle cannot progress from the G2 growth stage to the mitosis (M) stage. This leads to continuing cell growth without division, which presents as macrocytosis. Megaloblastic anemia has a rather slow onset, especially when compared to that of other anemias. The defect in red cell DNA synthesis is most often due to hypovitaminosis, specifically vitamin B12 deficiency or folate deficiency. Loss of micronutrients may also be a cause. Megaloblastic anemia not due to hypovitaminosis may be caused by antimetabolites that poison DNA production directly, such as some chemotherapeutic or antimicrobial agents (for example azathioprine or trimethoprim). The pathological state of megaloblastosis is characterized by many large immature and dysfunctional red blood cells (megaloblasts) in the bone marrow and also by hypersegmented neutrophils (defined as the presence of neutrophils with six or more lobes or the presence of more than 3% of neutrophils with at least five lobes). These hypersegmented neutrophils can be detected in the peripheral blood  (using a diagnostic smear of a blood sample).
Blood findings
The blood film can point towards vitamin deficiency:

Decreased red blood cell (RBC) count and hemoglobin levels
Increased mean corpuscular volume (MCV, >100 fL) and mean corpuscular hemoglobin (MCH)
Normal mean corpuscular hemoglobin concentration (MCHC, 32–36 g/dL)
Decreased reticulocyte count due to destruction of fragile and abnormal megaloblastic erythroid precursor. The platelet count may be reduced. Neutrophil granulocytes may show multisegmented nuclei ("senile neutrophil"). This is thought to be due to decreased production and a compensatory prolonged lifespan for circulating neutrophils, which increase numbers of nuclear segments with age. Anisocytosis (increased variation in RBC size) and poikilocytosis (abnormally shaped RBCs). Macrocytes (larger than normal RBCs) are present. Ovalocytes (oval-shaped RBCs) are present. Howell-Jolly bodies (chromosomal remnant) also present.Blood chemistries will also show:

An increased lactic acid dehydrogenase (LDH) level. The isozyme is LDH-2 which is typical of the serum and hematopoietic cells. Increased homocysteine and methylmalonic acid in Vitamin B12 deficiency
Increased homocysteine in folate deficiencyNormal levels of both methylmalonic acid and total homocysteine rule out clinically significant cobalamin deficiency with virtual certainty.Bone marrow (not normally checked in a patient suspected of megaloblastic anemia) shows megaloblastic hyperplasia. See also
List of circulatory system conditions
List of hematologic conditions

References
External links

GeneReview/NCBI/NIH/UW entry on Thiamine-Responsive Megaloblastic Anemia Syndrome
Rare Anemias Foundation