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Iron Deficiency Screening in Pregnancy: Ferritin Thresholds and Timing

By the Find Her Doctor editorial team · Updated November 1, 2026 · 6 min read

Screen every pregnant patient for anemia with a CBC at the first prenatal visit and again at 24–28 weeks, per ACOG. Add a serum ferritin early in pregnancy for anyone at risk of iron deficiency. Under the 2026 ASH diagnostic guideline, a ferritin of 30 µg/L (ng/mL) or lower now defines iron deficiency in pregnancy, matching the long-standing BSH cutoff. Hemoglobin alone finds iron deficiency late, after stores are already gone.

Why the CBC isn't enough

Iron deficiency progresses in stages: depleted stores (low ferritin), then iron-restricted erythropoiesis (low transferrin saturation), then anemia. Hemoglobin only moves at the last stage, and plasma volume expansion muddies it further through dilution.

Demand climbs steeply in the second and third trimesters, so a patient who starts with marginal stores can be anemic before her next scheduled CBC. US survey data cited in the 2024 USPSTF review show iron deficiency becoming more common with each trimester. The practical payoff of finding depleted stores early is time: oral iron has room to work, and fewer patients end up needing IV iron or transfusion near term. The familiar scenario of a 36-week Hb of 9 with no ferritin ever drawn is largely preventable.

Hemoglobin thresholds by trimester

ACOG Practice Bulletin 233 uses the CDC trimester-specific definitions. WHO's 2024 guideline uses the same values, with a separate second-trimester cutoff.

Two practice points:

• Altitude and smoking raise hemoglobin. WHO provides adjustments; in the New York metro area, smoking is the one that realistically applies.
• Race-specific thresholds appear in older literature. Many institutions now use one threshold for all patients, since a lower cutoff can leave iron deficiency in Black patients undiagnosed.

Choosing a ferritin cutoff

There's still no single universal number. The reference points:

Why 30 and not 15. A 12–15 µg/L cutoff is highly specific, but in pregnancy it misses a large share of patients whose stores won't cover the coming demand. A systematic review of ferritin thresholds in pregnancy found published cutoffs varied widely and few had been validated against bone marrow iron. The asymmetry is the argument: a false positive costs a few weeks of cheap oral iron; a false negative can mean anemia at delivery. ASH's 2026 diagnostic guideline raised the adult cutoff from 15 to 30 ng/mL and applies it to pregnant and menstruating patients, the threshold BSH has used in pregnancy since 2020. ASH also notes a cutoff of up to 50 ng/mL may be appropriate with symptoms or ongoing risk factors. Assays vary between labs, so values near the line deserve judgment.

The inflammation caveat. Ferritin is an acute-phase reactant and rises with infection, inflammatory disease and obesity-related inflammation. With inflammation present, the 2026 ASH guideline recommends checking ferritin and transferrin saturation together and considering iron deficiency if ferritin is below 100 ng/mL or TSAT is below 20%. See interpreting iron studies for a fuller framework.

A screening schedule that works in clinic

Consistent with ACOG and BSH:

1. First prenatal visit: CBC for everyone; ferritin for anyone at increased risk. Many practices just check ferritin universally, because the risk factors are so common in a typical New York panel.
2. 24–28 weeks: Repeat the CBC. Check ferritin if it wasn't done earlier, if the patient is anemic, or if she was iron-deficient and on treatment. Drawing it with the glucose screen saves a stick.
3. On treatment: Recheck hemoglobin about 2–4 weeks after starting oral iron. BSH expects a measurable rise in that window if the patient is adherent and absorbing.
4. Around 34–36 weeks: Consider a CBC for anyone who had anemia, low ferritin or poor adherence earlier, while there's still time for IV iron before delivery.

Higher-risk patients

• Short interpregnancy interval or multiple gestation
• Heavy menstrual bleeding before pregnancy
• Prior anemia or prior IV iron
• Vegetarian or vegan diet, or food insecurity
• Bariatric surgery, celiac disease or inflammatory bowel disease
• Hyperemesis or other conditions limiting oral intake
• Adolescent pregnancy
• Patients who decline blood products, where optimizing hemoglobin before delivery matters most

Reading the USPSTF 2024 I statement correctly

The USPSTF gave an I statement (insufficient evidence) for both routine screening and routine iron supplementation in asymptomatic pregnant people. It's sometimes read as a recommendation against screening. It isn't. The Task Force found no trials testing screening strategies against maternal and infant outcomes, which is an evidence gap, not evidence of harm. ACOG's CBC at the first visit and at 24–28 weeks still stands, and the CBC remains part of routine prenatal labs.

Interpreting first-visit results

ACOG recommends hemoglobinopathy screening with a CBC for all pregnant patients, and electrophoresis for those with microcytosis without iron deficiency or of high-risk ancestry. Given the city's mix of Mediterranean, South Asian, Southeast Asian, African and Caribbean ancestry, microcytosis with a normal ferritin comes up often. It shouldn't be treated with empiric iron indefinitely.

Treating what you find

• Oral iron first for most patients identified before the third trimester. Lower or alternate-day dosing improves tolerability and absorption, because a large dose raises hepcidin for about 24 hours. Counsel on separating iron from calcium, tea, coffee and antacids. Asking "How many doses did you actually take last week?" is often more useful than repeating the lab.
• Escalate to IV iron if hemoglobin doesn't respond, oral iron isn't tolerated, the patient presents late in the third trimester, or anemia is severe. BSH supports parenteral iron from the second trimester onward. See IV iron in pregnancy and when oral iron fails.
• Keep treating after hemoglobin normalizes. BSH advises continuing oral iron for about 3 months and until at least 6 weeks postpartum to rebuild stores.

Making it stick in your workflow

Screening only helps if someone acts on the result. Practices that do this well tend to:

• Build ferritin into the standard first-visit lab set
• Use a nurse- or MA-driven protocol to start oral iron and book the recheck
• Flag low third-trimester ferritin for escalation instead of waiting for the 36-week visit
• Have an agreed IV iron pathway that doesn't require a full hematology consult for uncomplicated iron deficiency

Our outpatient anemia protocol for OB practices has a template.

Referral partnership in New York

For persistent anemia despite oral iron, unexplained microcytosis, or a third-trimester deficit that needs rapid correction, a direct hematology pathway saves weeks. Premier Hematology & Oncology, which sponsors our professional resources, offers referral pathways, fast-track IV iron scheduling and co-management across the New York area. Details are on our hematology partners page.

Frequently asked questions

Sources

• ASH — 2026 Guidelines for Diagnosis of Iron Deficiency (Blood Advances)
• ACOG — Anemia in Pregnancy (Practice Bulletin No. 233)
• British Society for Haematology — UK guidelines on the management of iron deficiency in pregnancy (2020)
• WHO — Guideline on haemoglobin cutoffs to define anaemia in individuals and populations (2024)
• WHO — Guideline on use of ferritin concentrations to assess iron status in individuals and populations (2020)
• USPSTF — Screening and Supplementation for Iron Deficiency and Iron Deficiency Anemia During Pregnancy (2024)
• CDC — Recommendations to prevent and control iron deficiency in the United States

This article is for general education and is not medical advice. It does not replace an evaluation by a licensed clinician who knows your history. If you think you have a medical emergency, call 911.