A woman at 36 weeks gestation feels faint and nauseated while lying flat on her back during an antenatal scan. Her BP is recorded at 82/50 mmHg. Turning her onto her left side promptly relieves all symptoms. The most likely mechanism is:

• A Vasovagal stimulation from pressure on the pelvic brim
• B Compression of the abdominal aorta increasing afterload
• C Compression of the inferior vena cava reducing venous return ✓
• D Placental release of vasodilator prostaglandins

Explanation

In late pregnancy the gravid uterus compresses the inferior vena cava when the mother lies supine, cutting venous return and causing supine hypotension syndrome. Relief by the left lateral position, which displaces the uterus off the IVC, confirms the diagnosis. Aortic compression alone does not produce this degree of hypotension, and prostaglandin release is unrelated to posture. This is why imaging and procedures near term are performed with left uterine displacement.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.