What physiological change in pregnancy commonly lowers blood pressure in the early trimesters?

In early pregnancy, the phenomenon of decreased systemic vascular resistance significantly contributes to the reduction of blood pressure. As the placenta develops, the body undergoes adaptations that include the expansion of blood vessels (vasodilation) to accommodate increased blood flow to the placenta and developing fetus. This process decreases resistance in the peripheral vascular system, allowing blood to flow more easily, which can lower overall blood pressure.

This change in systemic vascular resistance contrasts with other factors such as increased cardiac output and increased blood volume, which typically rise during pregnancy. While increased cardiac output is a physiological adaptation meant to supply adequate blood flow to both the mother and fetus, it alone does not directly result in lowered blood pressure during the early stages of pregnancy; the effect of decreased vascular resistance is more influential.

Decreased blood viscosity plays a minor role in blood pressure regulation and is not as significant a factor in explaining the drop in blood pressure seen during the early trimesters. Consequently, the primary physiological change responsible for lowering blood pressure during this time is the decrease in systemic vascular resistance.