What are the potential causes of dizziness in a 22-year-old pregnant woman at 22 weeks gestation with a history of not taking iron supplements due to constipation and having stable blood pressure and normal pregnancy scans?

Causes of Dizziness in Pregnancy

In a 22-year-old pregnant woman at 22 weeks gestation with dizziness and a history of not taking iron supplements, iron deficiency anemia is the most likely cause and should be immediately evaluated and treated, even though her blood pressure is stable.

Primary Cause: Iron Deficiency Anemia

Why Iron Deficiency is the Leading Concern

• Iron deficiency causes symptoms before anemia develops: Impaired neurocognitive function, mood disturbances, and dizziness can occur even when hemoglobin remains above diagnostic thresholds for anemia 1
• High prevalence in mid-pregnancy: 18.6% of pregnant women have iron deficiency, with only 16.2% meeting criteria for anemia, meaning standard hemoglobin screening may miss the diagnosis 2, 1
• This patient has clear risk factors: Non-compliance with iron supplementation due to gastrointestinal side effects (constipation) places her at significantly increased risk 2
• Second trimester timing: Iron deficiency prevalence increases as pregnancy progresses, with rates climbing from 6.9% in first trimester to 29.5% in third trimester 2

Diagnostic Approach

• Screen with hemoglobin/hematocrit immediately: This is the first-line screening test, though it may miss iron deficiency without anemia 2, 3
• Anemia threshold at 22 weeks: Hemoglobin <11 g/dL defines anemia in all trimesters 3
• Consider serum ferritin if hemoglobin is normal: Low ferritin indicates iron deficiency even without anemia, though it has limitations as an acute phase reactant 2, 1
• If hemoglobin <9.0 g/dL or hematocrit <27.0%: Refer to a physician for further evaluation 2

Treatment Strategy

• For confirmed anemia (Hb <11 g/dL): Prescribe 60-120 mg elemental iron daily 2, 3
• For iron deficiency without anemia: Start 30 mg elemental iron daily 2, 3
• Address constipation concerns: Consider slow-release ferrous sulfate formulations for better tolerability, divided doses (100 mg/day split), or supplementing with vitamin C to enhance absorption 4
• Reassess in 4 weeks: Hemoglobin should increase by 1 g/dL or hematocrit by 3%; if not, evaluate for other causes including thalassemia minor or sickle cell trait 2

Secondary Causes to Consider

Physiologic Changes of Pregnancy

• Normal plasma volume expansion: Physiologic hemodilution occurs in pregnancy as plasma volume increases more rapidly than red cell mass, which can cause relative hypotension and dizziness 5
• Orthostatic hypotension: Constitutional and orthostatic hypotension are common causes of dizziness, particularly in pregnancy 6

Cardiovascular Causes

• Gestational hypertension and preeclampsia: While her blood pressure is currently stable, these conditions develop after 20 weeks gestation and can present with dizziness, headache, and visual disturbances 2, 7
• Arrhythmias: Supraventricular tachycardias can cause dizziness, though less common 2

Other Nutritional Deficiencies

• Vitamin B12 or folate deficiency: Can cause macrocytic anemia and neurologic symptoms including dizziness 5

Critical Pitfalls to Avoid

• Don't dismiss symptoms with "normal" hemoglobin: Iron deficiency symptoms occur before anemia develops, and standard screening misses iron deficiency without anemia 1
• Don't accept non-compliance: Address the constipation issue directly with alternative iron formulations or adjunctive measures rather than leaving iron deficiency untreated 4
• Don't overlook severe anemia: If hemoglobin is <9 g/dL, this requires physician referral as it carries significant risks including intrauterine growth restriction, prematurity, and increased peripartum blood loss 2, 8
• Don't assume stable blood pressure rules out preeclampsia: Monitor for other signs including proteinuria, right upper quadrant pain, headache, visual changes, and hyperreflexia 2, 7

Maternal and Fetal Risks if Untreated

• Maternal complications: Increased risk of postpartum hemorrhage, need for transfusion, prolonged labor, and urgent cesarean section 9
• Fetal complications: Low birth weight, small-for-gestational-age, preterm delivery, stillbirth, and early neonatal death 1, 9
• Neurodevelopmental effects: Impaired fetal brain development even with maternal iron deficiency without anemia 1