Situation: The nurse works in the prenatal clinic and antepartum ward of a provincial hospital. Using the current World Health Organization (WHO) hemoglobin cut-offs for anemia in pregnancy, a client in the second trimester is considered anemic when her hemoglobin is below which level?

Because plasma volume rises more than red cell mass, hemoglobin normally falls most in the second trimester. Anemia is defined as hemoglobin below 11.0 g/dL (110 g/L) in the first and third trimesters and below 10.5 g/dL (105 g/L) in the second trimester.

Hemodilution and the trimester-specific definition of anemia

During pregnancy, maternal plasma volume expands by roughly 40–50% while red cell mass increases only about 20–30%. This disproportionate expansion produces physiologic hemodilution, so hemoglobin normally declines even when iron stores and erythropoiesis are adequate. The decline is most pronounced in the second trimester, when plasma volume expansion peaks before red cell mass catches up in the third trimester. Because a falling hemoglobin in the second trimester may therefore reflect normal dilution rather than true anemia, the diagnostic threshold is set lower during this period.

The World Health Organization defines anemia in pregnancy using trimester-specific cutoffs. In the first and third trimesters, anemia is diagnosed when hemoglobin falls below 11.0 g/dL (110 g/L). In the second trimester, the cutoff is lowered to 10.5 g/dL (105 g/L) to account for the expected nadir of physiologic dilution. A second-trimester hemoglobin below 10.5 g/dL therefore signals anemia requiring evaluation, whereas a value between 10.5 and 11.0 g/dL may still represent normal pregnancy hemodilution.

The 2024 WHO revision reaffirmed the second-trimester pregnancy cutoff while adjusting thresholds for young children and high-altitude residents, indicating that the pregnancy-specific value remains the current standard ^[1]. A cross-sectional study that used 105 g/L as the trigger for anemia workup in the second trimester found that many women below this threshold had a positive evaluation for nondilutional anemia, supporting the clinical utility of this cutoff for identifying true pathology rather than physiologic change ^[2].

Trimester	WHO anemia cutoff	Rationale
First trimester	11.0 g/dL (110 g/L)	Plasma volume expansion is minimal; hemoglobin near nonpregnant baseline
Second trimester	10.5 g/dL (105 g/L)	Peak hemodilution; lower threshold avoids overdiagnosis of physiologic decline
Third trimester	11.0 g/dL (110 g/L)	Red cell mass expansion catches up; hemoglobin rises from second-trimester nadir

Watch out! A hemoglobin of 10.8 g/dL in the second trimester is not classified as anemia under WHO criteria, even though it would be anemic in the first or third trimester. Applying a single 11.0 g/dL cutoff across all trimesters leads to overdiagnosis and unnecessary iron supplementation during the physiologic nadir.

Key point! The second-trimester cutoff of 10.5 g/dL exists specifically because hemodilution is maximal at this stage. When interpreting a prenatal hemoglobin result, always match the value to the correct trimester-specific reference before labeling the client anemic or initiating an anemia workup.