Why does the supine position lead to decreased uteroplacental blood flow after 20 weeks of gestation?

The supine position leads to decreased uteroplacental blood flow after 20 weeks of gestation primarily because the aorta and vena cava become compressed. When a pregnant individual lies flat on their back, the weight of the growing uterus can exert pressure on these major blood vessels. This compression restricts blood flow to and from the heart, leading to reduced venous return and subsequent lower cardiac output, which in turn can affect placental perfusion. Ensuring adequate blood flow to the placenta is crucial for fetal nourishment and oxygenation, and any position that compromises this flow can have negative implications for fetal well-being.

The other options do not accurately capture the primary mechanism at play in this scenario. Fluid imbalance may affect circulation but is not the direct reason for decreased uteroplacental blood flow in the supine position. Additionally, the supine position does not inherently lead to decreased maternal heart rate; rather, heart rate may remain relatively unchanged or be influenced by other factors. Finally, while increased uterine tone can occur due to various reasons during pregnancy, it is not a direct consequence of the supine position that directly impacts blood flow to the uterus and placenta.