What is the recommended diagnostic work‑up for anemia in a pregnant woman, including initial CBC, iron studies, folate and vitamin B12 assessment, hemoglobin electrophoresis, and other relevant tests?

Diagnostic Approach to Anemia in Pregnancy

Screen all pregnant women with a complete blood count (CBC) at the first prenatal visit and again at 28 weeks gestation, using hemoglobin <11 g/dL as the diagnostic threshold for anemia across all trimesters. 1

Initial Screening Strategy

Begin with hemoglobin and hematocrit measurement as the first-line screening test 2, 3. While the USPSTF found insufficient evidence to recommend for or against routine screening 2, other major organizations including the CDC, Institute of Medicine, and ACOG all recommend universal screening 2. Given the high prevalence (18.6% of pregnant women have iron deficiency, with 16.2% having concurrent anemia) and potential for serious maternal and fetal complications, screening is the prudent approach in real-world practice 3.

Timing of Screening

• First trimester: Initial CBC at booking visit
• Third trimester: Repeat CBC at 28 weeks
• Additional screening may be warranted if planning cesarean delivery or anticipating complicated delivery with blood loss 3

Diagnostic Work-Up When Anemia is Detected

Step 1: Confirm Iron Deficiency

When hemoglobin is <11 g/dL, obtain iron studies including serum ferritin 4. Iron deficiency is diagnosed when:

• Serum ferritin <30 μg/L 5
• This is the most reliable single test for iron deficiency in pregnancy

Important caveat: Ferritin is an acute phase reactant and may be falsely elevated in the presence of inflammation, infection, or chronic disease 2. In these contexts, ferritin levels may appear normal despite true iron deficiency.

Step 2: Evaluate Red Blood Cell Indices

The CBC provides critical information:

• Mean corpuscular volume (MCV): Low MCV suggests iron deficiency or thalassemia
• Red blood cell distribution width (RDW): Elevated in iron deficiency
• Reticulocyte hemoglobin content (RET-He) correlates well with MCV and can help assess iron status 6

Step 3: Consider Other Nutritional Deficiencies

Folate assessment is generally not necessary as a routine test because:

• Folate deficiency in pregnancy is rare 7
• Most pregnant women receive prenatal vitamins containing folic acid
• Neural tube defect prevention requires periconceptional supplementation, making late pregnancy testing less clinically useful

Vitamin B12 (cobalamin) assessment should be obtained when:

• MCV is elevated (macrocytic anemia)
• Patient has history of bariatric surgery
• Patient follows strict vegetarian/vegan diet
• Neurological symptoms are present
• Iron deficiency has been excluded 8

The rising prevalence of obesity and bariatric surgery makes B12 deficiency increasingly relevant, as low maternal cobalamin associates with fetal growth retardation and metabolic complications 8.

Step 4: Hemoglobinopathy Screening

Obtain hemoglobin electrophoresis when:

• Patient belongs to high-risk ethnic groups (African, Mediterranean, Middle Eastern, Southeast Asian descent)
• MCV is disproportionately low relative to degree of anemia
• Anemia fails to respond to iron therapy
• Family history of hemoglobinopathies exists 1

This should ideally be done early in pregnancy or preconceptionally to allow for partner testing and genetic counseling.

Risk Factors Requiring Enhanced Surveillance

Certain populations warrant closer monitoring 2, 3:

Dietary factors:

• Vegetarian/vegan diet (especially lacking iron-rich plant sources)
• Diet low in heme iron sources

Medical factors:

• Gastrointestinal disease affecting absorption
• Medications reducing iron absorption (antacids, proton pump inhibitors)
• Short interpregnancy interval (<18 months)

Demographic factors:

• Non-Hispanic Black and Mexican American women (higher baseline prevalence)
• Multiparity (≥2 previous pregnancies)
• History of heavy menstrual bleeding

Practical Algorithm

1. All pregnant women: CBC at first visit and 28 weeks
2. If Hb <11 g/dL: Check serum ferritin
3. If ferritin <30 μg/L: Diagnose iron deficiency anemia → treat
4. If ferritin normal but anemia persists: Consider inflammation masking iron deficiency, or evaluate for:
   • B12 deficiency (if macrocytic)
   • Hemoglobinopathy (if microcytic, family history, or ethnic risk factors)
   • Chronic disease/inflammation
5. If no clear cause identified: Consider hematology referral

Common Pitfalls to Avoid

• Don't rely solely on ferritin in the presence of infection or inflammation – it will be falsely reassuring
• Don't assume all anemia in pregnancy is iron deficiency – while it's the most common cause (affecting up to 50% globally), hemoglobinopathies and B12 deficiency must be excluded when appropriate 1, 8
• Don't wait until third trimester to screen – early anemia (first/second trimester) associates with worse fetal outcomes than late anemia 2
• Don't forget physiologic hemodilution – plasma volume expansion in pregnancy can lower hemoglobin concentration without true anemia, but the 11 g/dL threshold accounts for this 3, 1

Evidence Quality Considerations

The USPSTF guidelines 2 represent high-quality systematic reviews but found insufficient evidence for screening benefits on maternal/fetal outcomes. However, this reflects gaps in research rather than evidence of harm or lack of benefit. The more recent FIGO recommendations 1 from 2025 provide the most current guidance supporting universal screening, acknowledging that anemia contributes to over 115,000 maternal deaths annually worldwide. In clinical practice, the consensus approach favoring screening is justified given the high prevalence, low screening cost/risk, and availability of effective treatment.