In plain words
Hemoglobin is the protein inside red blood cells. It carries oxygen from the lungs to the rest of the body. A blood test measures how much is present. A low level is called anemia. In pregnancy, the level is read against cut-offs that change by trimester, because blood volume rises. After birth, levels are often checked again after blood loss. Iron is needed to make hemoglobin, so is the most common cause of a low level worldwide. A low level can bring tiredness, breathlessness, dizziness, or a fast heartbeat. Some people have no symptoms. A number on a lab report is a starting point for a conversation, not a full picture.
The deeper version
Hemoglobin is a tetrameric, heme-containing protein in erythrocytes that binds oxygen cooperatively. Anemia in pregnancy is defined by trimester-specific hemoglobin thresholds that account for physiologic , following WHO criteria (2011). The first and third trimester thresholds sit near 11 grams per deciliter and the second trimester near 10.5. Iron deficiency, with or without anemia, is the leading cause worldwide. Hemoglobin alone does not separate iron deficiency from other causes, so , , and reticulocyte indices are used alongside it. Postpartum, hemoglobin is measured after hemorrhage or before discharge, and the recovery trajectory guides iron decisions. The main controversy concerns the level below which treatment is clearly beneficial in people without symptoms, and whether the trimester cut-offs apply equally across ancestry and altitude.
Where it comes up
- Low iron and anemiaPregnancy: Your Body
- Blood-building foods after birthNourishment and Traditions
- Anemia and exhaustion after birthPostpartum: Your Body
What mothers here say about hemoglobin
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Sources
- World Health Organization — Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity (2011)
- American College of Obstetricians and Gynecologists — Practice Bulletin 233: Anemia in pregnancy (2021)