High Altitude Does Not Cause Decreased Hemoglobin Levels
High altitude actually increases hemoglobin levels rather than decreasing them, making it the condition that will NOT cause decreased Hb levels or anemia. 1, 2
Why High Altitude Increases Hemoglobin
At elevations above 3,000 feet, the body compensates for lower oxygen availability by producing more red blood cells, which raises hemoglobin concentration. 2
Hemoglobin values must be adjusted upward when screening for anemia in people living at high altitudes—for example, at 2,500 feet elevation, add 1.3 g/dL to the measured value; at 3,000 feet add 1.9 g/dL; and at 4,000 feet add 3.5 g/dL. 1
This physiologic adaptation means that a person at high altitude with a "normal" hemoglobin by sea-level standards may actually be anemic when altitude-adjusted cutoffs are applied. 2
Conditions That DO Cause Decreased Hemoglobin
Pregnancy Causes Physiologic Anemia
Pregnancy decreases hemoglobin concentration during the first and second trimesters primarily due to hemodilution from expanding blood volume. 1, 2
Anemia in pregnancy is defined as hemoglobin <11.0 g/dL in the first trimester, <10.5 g/dL in the second trimester, and <11.0 g/dL in the third trimester. 2, 3
Without iron supplementation, hemoglobin remains low throughout the third trimester, but with adequate iron intake it gradually rises toward pre-pregnancy levels. 1, 2
Iron deficiency compounds pregnancy anemia because menstrual losses average 0.3-0.5 mg/day and pregnancy depletes maternal iron stores. 1, 4
Nutritional Deficiencies Cause True Anemia
Iron deficiency anemia accounts for approximately 75% of true anemias during pregnancy and is the most common nutritional cause of decreased hemoglobin worldwide. 5, 6
Folate deficiency causes megaloblastic anemia, particularly in women with inadequate diets who are not receiving prenatal supplements. 5, 6
Vitamin B12 deficiency, though rare in pregnancy, can also cause anemia and may result in congenital abnormalities due to its role in embryogenesis. 5
Nutritional iron deficiency anemia affects more than 56 million women globally, with two-thirds being from Asia. 5
Critical Clinical Pitfall
Do not confuse the physiologic hemodilution of pregnancy with true anemia requiring aggressive treatment. The naturally lower hemoglobin in pregnancy reflects expanded blood volume, not necessarily pathologic red blood cell deficiency. However, iron supplementation (30 mg/day starting at first prenatal visit) is still recommended for all pregnant women to prevent true iron deficiency anemia. 2