Identifying and Managing Iron‑Deficiency Anemia in the Third Trimester

Iron‑deficiency anemia (IDA) is one of the most common hematologic complications encountered during the third trimester of pregnancy. By this stage, the fetal demand for iron peaks, maternal blood volume has expanded dramatically, and the physiological changes of pregnancy place additional stress on the mother’s iron stores. When these demands outpace dietary intake and mobilizable reserves, hemoglobin synthesis can falter, leading to a cascade of maternal‑ and fetal‑related risks. Understanding how to recognize, evaluate, and treat IDA in the final weeks of gestation is essential for obstetric providers, midwives, and allied health professionals who aim to safeguard both maternal health and neonatal outcomes.

Understanding Iron‑Deficiency Anemia in Late Pregnancy

Iron‑deficiency anemia is defined by a reduction in hemoglobin concentration that results from insufficient iron to support normal erythropoiesis. In the third trimester, the World Health Organization (WHO) and most national guidelines consider a hemoglobin level < 11 g/dL (or < 10.5 g/dL in some high‑altitude settings) as diagnostic of anemia, with levels < 7 g/dL indicating severe disease. The prevalence of IDA rises sharply after 28 weeks, affecting up to 30 % of pregnant women in low‑resource settings and 10–15 % in high‑income countries.

Key physiological contributors include:

• Expanded plasma volume – a 40–50 % increase dilutes red blood cells, lowering the concentration of hemoglobin (physiologic anemia).
• Fetal iron sequestration – the growing fetus extracts iron from the maternal circulation, especially during the third trimester when iron accretion in fetal stores peaks.
• Inadequate dietary intake or absorption – even modest deficits become clinically relevant when maternal stores are already depleted.
• Pre‑existing depletion – women who entered pregnancy with low ferritin (< 15 µg/L) are at heightened risk.

Pathophysiology of Iron Deficiency in the Third Trimester

The cascade begins with depletion of hepatic iron stores, reflected by low serum ferritin. As stores dwindle, transferrin saturation falls, prompting the bone marrow to reduce erythropoietic output. The resulting microcytic, hypochromic red cells have a shortened lifespan, further compromising oxygen delivery. In the placenta, iron deficiency can impair trophoblast function, potentially reducing nutrient transfer and contributing to intrauterine growth restriction (IUGR). Moreover, maternal hypoxia can trigger compensatory increases in cardiac output, placing additional strain on the cardiovascular system.

Clinical Presentation and Red Flags

While many women with mild IDA are asymptomatic, the third trimester can unmask classic signs:

• Fatigue and dyspnea disproportionate to gestational age.
• Pallor of the conjunctivae, nail beds, and mucous membranes.
• Palpitations or tachycardia at rest.
• Dizziness or presyncope, especially when standing.
• Exertional chest discomfort – a warning sign for severe anemia.

Red‑flag symptoms that warrant urgent evaluation include:

• Persistent chest pain or pressure.
• Syncope or near‑syncope episodes.
• New‑onset orthostatic hypotension.
• Rapidly worsening fatigue interfering with daily activities.

These presentations may signal severe anemia (Hb < 7 g/dL) or an underlying cardiac compromise, both of which demand immediate medical attention.

Laboratory Evaluation and Diagnostic Criteria

A systematic laboratory work‑up confirms iron deficiency and excludes alternative etiologies:

When ferritin is borderline, the combination of low transferrin saturation and elevated TIBC strengthens the diagnosis. In cases where anemia persists despite normal iron studies, clinicians should consider alternative causes such as folate deficiency, vitamin B12 deficiency, hemoglobinopathies, or chronic disease.

Differential Diagnosis: Beyond Iron Deficiency

Although iron deficiency is the most common cause, the third trimester can unmask other hematologic disorders:

• Anemia of chronic disease – often associated with inflammatory conditions; ferritin may be normal or high.
• Folate or vitamin B12 deficiency – macrocytic picture, elevated MCV.
• Thalassemia trait – microcytosis with normal iron studies, family history.
• Hemolytic anemias – elevated lactate dehydrogenase (LDH), indirect bilirubin, and reticulocyte count.

A thorough history (dietary patterns, chronic illnesses, medication use) and targeted labs help delineate these entities.

Evidence‑Based Management Strategies

Pharmacologic Therapy: Oral vs Intravenous Iron

Oral Iron

First‑line therapy for most third‑trimester IDA, provided the patient can tolerate the regimen and there are no contraindications (e.g., malabsorption, severe anemia requiring rapid correction). Common preparations include ferrous sulfate, ferrous gluconate, and ferrous fumarate.

Intravenous (IV) Iron

Indicated when:

• Oral iron is ineffective after 2–4 weeks (persistent low ferritin or Hb).
• Intolerance to oral iron (gastrointestinal side effects).
• Severe anemia (Hb < 8 g/dL) where rapid repletion is desired.
• Malabsorption syndromes (e.g., celiac disease, bariatric surgery).

IV formulations such as iron sucrose, ferric carboxymaltose, and iron dextran have demonstrated safety in pregnancy when administered under obstetric supervision. The total dose required can be calculated using the Ganzoni formula or simplified dosing tables based on weight and target Hb.

Selecting the Appropriate Iron Form

• Ferrous salts (oral) provide elemental iron in a readily absorbable form but are associated with gastrointestinal irritation.
• Polysaccharide‑iron complexes (e.g., iron polymaltose) may have a more favorable side‑effect profile, though evidence for superior efficacy is limited.
• IV iron complexes differ in molecular weight and stability; ferric carboxymaltose allows larger single‑dose infusions (up to 1000 mg) with a low risk of anaphylaxis, making it convenient for late‑pregnancy visits.

Dosing Considerations and Treatment Duration

• Oral iron: 100–200 mg elemental iron daily, divided into two doses if gastrointestinal tolerance is an issue. Treatment should continue for at least 3 months after achieving target Hb to replenish stores.
• IV iron: Total cumulative dose typically ranges from 500–1000 mg, administered over 1–2 sessions. Post‑infusion, a repeat ferritin measurement at 2–4 weeks guides the need for additional dosing.

Managing Side Effects and Enhancing Tolerability

• Gastrointestinal discomfort – advise taking the tablet with a small amount of food (despite modest reduction in absorption) and using a lower‑dose, slower‑release formulation if needed.
• Constipation – increase dietary fiber, fluid intake, and consider safe stool softeners (e.g., docusate sodium).
• Dark stools – reassure patients that this is a benign effect of unabsorbed iron.

For IV iron, monitor vital signs during and after infusion; rare hypersensitivity reactions can be mitigated by having emergency medications (e.g., epinephrine) readily available.

Integrating Anemia Management into Prenatal Care

A coordinated approach ensures that iron therapy does not interfere with other aspects of prenatal care:

1. Timing of Visits – Align iron assessments with routine third‑trimester labs (e.g., glucose tolerance test, group B streptococcus screening) to minimize additional appointments.
2. Interdisciplinary Communication – Share iron status updates with the obstetrician, midwife, and, when appropriate, the nutritionist to maintain a unified care plan.
3. Medication Reconciliation – Review concurrent medications (e.g., antacids, calcium supplements) that may impair iron absorption and adjust timing accordingly.

Follow‑Up Protocols and Monitoring Treatment Response

• Initial reassessment – 2–3 weeks after initiating oral iron, repeat hemoglobin and ferritin. A rise of ≥ 1 g/dL in Hb suggests adequate response.
• Mid‑course evaluation – At 6 weeks, ensure ferritin has risen above 30 µg/L; if not, consider dose escalation or transition to IV iron.
• Pre‑delivery check – Within 2 weeks of the anticipated delivery date, confirm that hemoglobin is ≥ 11 g/dL to reduce the risk of peripartum transfusion.

If the patient fails to demonstrate an appropriate hematologic response, revisit adherence, gastrointestinal tolerance, and potential malabsorption. Referral to a hematologist may be warranted for complex cases.

Special Situations

Multiple Gestation – Twin or higher-order pregnancies increase iron demand by ~ 30 %. Early identification and more aggressive supplementation (often IV) are frequently required.

Pre‑Existing Hematologic Disorders – Women with sickle cell disease, thalassemia trait, or chronic kidney disease may have overlapping anemia etiologies; individualized treatment plans should be crafted in consultation with specialists.

Post‑Partum Transition – After delivery, iron stores are often further depleted due to blood loss. Continue iron therapy for at least 6 weeks postpartum, especially in breastfeeding mothers, to restore maternal reserves and support lactation.

Collaborative Care: Role of the Multidisciplinary Team

• Obstetrician/Midwife – Primary diagnostician, prescriber of therapy, and monitor of maternal‑fetal well‑being.
• Nurse Practitioner or Certified Nurse‑Midwife – Provides education, assesses adherence, and coordinates follow‑up visits.
• Pharmacist – Reviews drug interactions, advises on formulation selection, and assists with IV iron administration protocols.
• Dietitian/Nutritionist – While not the focus of this article, they can reinforce dietary strategies that complement pharmacologic therapy without duplicating content covered elsewhere.
• Hematologist – Consulted for refractory anemia, severe cases, or when alternative diagnoses are suspected.

Effective teamwork ensures that iron‑deficiency anemia is addressed promptly, safely, and in a manner that aligns with the overall prenatal care plan.

Patient Education and Counseling Points

• Explain the purpose of treatment – Emphasize that correcting anemia improves oxygen delivery to both mother and baby, reduces fatigue, and lowers the risk of complications during labor.
• Set realistic expectations – Hemoglobin may rise slowly; adherence to the regimen is crucial for long‑term success.
• Discuss side‑effect management – Provide practical tips for minimizing gastrointestinal discomfort and reassure patients about harmless changes such as dark stools.
• Highlight warning signs – Instruct patients to seek immediate care if they experience chest pain, severe shortness of breath, or fainting.
• Reinforce follow‑up – Stress the importance of scheduled lab checks and the possibility of dose adjustments based on results.

Clear, compassionate communication empowers expectant mothers to participate actively in their care, ultimately leading to better outcomes for both mother and child.

By systematically identifying iron‑deficiency anemia, employing evidence‑based therapeutic strategies, and integrating management into the broader framework of third‑trimester prenatal care, clinicians can effectively mitigate the risks associated with this common yet treatable condition. The result is a healthier pregnancy journey and a stronger start for the newborn.