Why did circulation return during CPR on a pregnant woman after displacing her uterus?

Circulation returned during CPR on a pregnant woman after displacing her uterus primarily because it relieved pressure from major blood vessels. During pregnancy, especially in the later stages, the weight of the uterus can compress the inferior vena cava and aorta when the woman is in a supine position. This compression can significantly impede venous return to the heart and compromise circulation, leading to decreased blood flow to the fetus and mother.

By displacing the uterus to the left (often referred to as left uterine displacement), this pressure is alleviated, allowing blood to flow more freely back to the heart. This increased venous return can help restore cardiac output and improve overall circulation, which is critical during CPR. Thus, ensuring that major blood vessels are not compressed is essential for effective resuscitation efforts in pregnant patients.