In plain words
Hemodilution is the thinning of blood that happens in pregnancy. The liquid part of blood, called plasma, grows a lot. Red blood cells grow too, but more slowly. The result is blood with a lower concentration of . This is normal. It is one reason blood tests in pregnancy use different cut-offs than tests at other times. Hemodilution explains part of the drop in hemoglobin that many people see in the second trimester. It does not explain all of it, since iron needs also rise. A low reading still deserves a look, because hemodilution can hide or mimic anemia.
The deeper version
Plasma volume expands by 40 to 50 percent and red cell mass by 20 to 30 percent over pregnancy. This produces physiologic hemodilution, with a nadir in hemoglobin concentration around the second trimester. It explains the fall in hemoglobin concentration; the iron requirement rises separately. Trimester-specific hemoglobin thresholds, such as those in WHO guidance (2011), correct for hemodilution when defining anemia. Hemodilution can obscure true , so and other markers are often used alongside hemoglobin. The controversy concerns the size of the correction used across populations, including people living at altitude, people who smoke, and people of different ancestry.
Where it comes up
- Low iron and anemiaPregnancy: Your Body
- Blood-building foods after birthNourishment and Traditions
- Blood sugar, dizziness, and energy crashesPregnancy: Eating
- What to eat in pregnancy: the short versionPregnancy: Eating
What mothers here say about hemodilution
No one has posted under this term yet.
Sources
- World Health Organization — Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity (2011)
- Cunningham FG and colleagues — Williams Obstetrics, 26th edition (2022)