Correct Answer: A. IVC compression
At 36 weeks of gestation, the gravid uterus has reached significant size and weight. When the patient lies supine (on her back), the enlarged uterus compresses the inferior vena cava (IVC) against the vertebral column, obstructing venous return from the lower body and pelvis. This leads to decreased cardiac preload, reduced cardiac output, and subsequent hypotension—manifesting as lightheadedness and dizziness. This is the classic supine hypotensive syndrome of pregnancy (also called aortocaval compression syndrome). The symptom resolution when lying on her side occurs because lateral recumbency shifts the uterine weight away from the IVC, restoring normal venous return. Ambulation also relieves symptoms by promoting venous return through muscle pump activity in the legs. This is a well-recognized physiological phenomenon in the third trimester and is a key reason why pregnant women are advised to avoid prolonged supine positioning, especially during labor and delivery. The timing (36 weeks) and positional nature of symptoms are pathognomonic for IVC compression.
Why the other options are wrong
B. Heavy meals — Heavy meals cause postprandial hypotension through splanchnic vasodilation and blood pooling in the GI tract, but this is not positionally dependent and would not be relieved by lying on the side or walking. The symptom pattern here is strictly positional (supine → symptomatic; lateral/upright → asymptomatic), which is inconsistent with meal-related hypotension. NBE includes this as a distractor for students who confuse general pregnancy-related nausea/dizziness with specific hemodynamic causes. C. Increased intracranial pressure — Increased ICP presents with headache, visual disturbances, and neurological signs—not positional lightheadedness that resolves with position change. ICP elevation in pregnancy is rare and would not be relieved by lateral recumbency. This is a trap for students who see 'dizziness' and think of neurological causes without considering the clear positional trigger and relief pattern that points to a hemodynamic mechanism. D. Excessive venous pooling at the feet — While venous pooling can occur in pregnancy due to increased venous capacitance and reduced lower-limb venous tone, it causes orthostatic hypotension (symptoms on standing, relief on lying down). This patient's symptoms are the opposite—she is symptomatic when supine and relieved by standing/walking. This reversal of the typical orthostatic pattern is the key discriminator against this option.
High-Yield Facts
- Supine hypotensive syndrome occurs in ~8–10% of pregnant women in the third trimester due to IVC compression by the gravid uterus.
- Lateral recumbency (left or right side-lying) is the first-line management and immediately relieves symptoms by shifting uterine weight off the IVC.
- Aortocaval compression can reduce cardiac output by up to 25–30% in supine position, explaining the hemodynamic collapse and syncope risk.
- Timing: IVC compression symptoms typically emerge after 24–28 weeks but are most common in the third trimester (28–36+ weeks).
- Clinical pearl: Pregnant women in labor should never be positioned supine for prolonged periods; left lateral tilt is standard to prevent aortocaval compression and fetal hypoxia.
Mnemonics
SOS for Supine Symptoms Supine → Obstruction of Superior venous return (IVC). When pregnant woman lies flat, uterus compresses IVC → ↓ venous return → ↓ CO → hypotension & dizziness. Side-lying = Solution. LAT = Left Anterior Tilt In labor/delivery, use Left Anterior Tilt (or any lateral position) to avoid aortocaval compression. Supine = Syndrome; Lateral = Life-saver.
NBE Trap
NBE pairs 'dizziness in pregnancy' with intracranial pathology (option C) to trap students who default to neurological thinking without analyzing the positional pattern. The key discriminator is that ICP-related symptoms do NOT resolve with position change, whereas this patient's symptoms are entirely positional—a classic hemodynamic, not neurological, presentation.
Clinical Pearl
In Indian obstetric practice, supine hypotensive syndrome is a leading cause of maternal syncope in the third trimester and during labor. Every pregnant woman presenting with positional dizziness after 28 weeks should be counseled to avoid supine rest and maintain left lateral decubitus position—this simple measure prevents both maternal symptoms and fetal hypoxia during labor, reducing the need for emergency interventions.
_Reference: DC Dutta's Textbook of Obstetrics (3rd edn), Ch. 8 (Maternal Physiology); Williams Obstetrics (25th edn), Ch. 4 (Maternal Physiology)_