What is the likely cause of dizziness in a 22-year-old pregnant woman at 22 weeks gestation, with stable blood pressure, normal pregnancy scans, and episodes of dizziness lasting about 10 minutes after meals, who has not been taking her prescribed iron tablets due to constipation?

Postprandial Hypotension and Iron Deficiency as Likely Causes

The most likely cause of this patient's dizziness is postprandial hypotension (low blood pressure after meals), potentially exacerbated by undiagnosed iron deficiency anemia from non-compliance with iron supplementation. 1, 2

Primary Mechanism: Postprandial Hypotension

The timing of symptoms—occurring specifically after meals and lasting approximately 10 minutes—is pathognomonic for postprandial hypotension, a common phenomenon in pregnancy. 2

• Postprandial hypotension occurs when blood is redistributed to the gastrointestinal tract during digestion, reducing cerebral perfusion 3
• Pregnancy physiologically predisposes women to hypotensive symptoms due to increased blood volume demands and hormonal changes affecting vascular tone 2
• The patient's stable blood pressure readings likely represent measurements taken outside of symptomatic episodes, missing the postprandial drops 3
• Classic symptoms include dizziness, lightheadedness, and fatigue—all consistent with this presentation 3, 4

Contributing Factor: Iron Deficiency Anemia

Non-compliance with iron supplementation significantly increases her risk of iron deficiency anemia, which independently causes dizziness and fatigue. 5

• Iron deficiency affects 18.6% of pregnant women, with 16.2% also having anemia 5
• At 22 weeks gestation, she is in the second trimester when iron deficiency prevalence increases 5
• Symptoms of iron deficiency include fatigue and dizziness, which overlap with her presentation 2
• The constipation side effect she's avoiding is well-documented with oral iron supplementation 1, 6

Critical Next Steps

Check hemoglobin/hematocrit and serum ferritin levels immediately to assess for iron deficiency anemia. 5

• Measurement of serum hemoglobin or hematocrit is the first-line screening test 5
• This will definitively establish whether iron deficiency is contributing to her symptoms 5

Address the iron supplementation non-compliance with alternative strategies:

• Switch to every-other-day dosing (rather than daily) to reduce gastrointestinal side effects including constipation 6
• Add a stool softener prophylactically—this is critical as iron-induced constipation can lead to serious complications including bowel obstruction 7
• Consider lower-dose formulations (30 mg elemental iron) if she's not anemic, or standard doses (60-120 mg) if anemia is confirmed 5
• Concurrent use of bulk-forming agents like psyllium husk or osmotic laxatives like polyethylene glycol are safe in pregnancy and effective for preventing constipation 5

Management of Postprandial Hypotension

Implement non-pharmacologic measures to prevent postprandial blood pressure drops:

• Advise smaller, more frequent meals rather than large meals to reduce splanchnic blood pooling 3, 4
• Recommend remaining seated or semi-recumbent for 30-60 minutes after eating 3
• Ensure adequate hydration before meals (you've already advised this appropriately) 3
• Avoid rapid position changes after eating 3, 4

Important Caveats

Orthostatic hypotension must be formally assessed if symptoms persist despite these interventions. 3

• Measure blood pressure supine, then after 1 and 3 minutes of standing 3
• A drop of ≥20 mmHg systolic or ≥10 mmHg diastolic defines orthostatic hypotension 3
• Pregnancy-related hypotensive symptoms are most common in early pregnancy and typically decrease toward term, so persistence beyond 24-28 weeks warrants further investigation 2

Rule out other causes if the clinical picture changes:

• New onset of room spinning (vertigo) would suggest vestibular pathology rather than hypotension 3
• Development of severe headache, visual changes, or blood pressure ≥140/90 mmHg would require immediate evaluation for preeclampsia 5, 8
• Palpitations or chest pain would necessitate cardiac evaluation 3

The combination of postprandial timing, stable baseline blood pressure, and iron non-compliance makes this a straightforward diagnosis requiring iron status assessment and dietary/lifestyle modifications rather than extensive workup. 2, 3