A 36-week pregnant woman feels dizzy and nauseated when lying flat on her back during an ultrasound examination. Her blood pressure drops to 88/54 mmHg. The symptoms resolve when she is turned to the left lateral position. The underlying mechanism is:
- A Compression of the abdominal aorta by the gravid uterus, reducing uterine perfusion
- B Compression of the inferior vena cava by the gravid uterus, reducing venous return ✓
- C Excessive baroreceptor reflex activation from expanded plasma volume
- D Placental release of vasodilatory prostaglandins into the maternal circulation
Correct answer: B. Compression of the inferior vena cava by the gravid uterus, reducing venous return
Explanation
In the supine position the gravid uterus compresses the inferior vena cava, reducing venous return and hence cardiac output, producing the supine hypotension syndrome relieved by lateral tilt. Aortic compression reduces uteroplacental perfusion rather than maternal systemic pressure and does not explain relief with positioning. Baroreceptor overactivity and prostaglandin excess are not recognized mechanisms of this syndrome.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.