During pregnancy, it is common for maternal blood pressure (BP) to decrease, especially in the second trimester.
This decrease happens because progesterone, a major pregnancy hormone, has a relaxing effect on smooth muscle.
Relaxation of smooth muscles in the walls of blood vessels leads to vasodilation.
Vasodilation lowers peripheral resistance, which results in a drop in maternal blood pressure.
Therefore, both the Assertion (A) and Reason (R) are correct.
Additionally, (R) correctly explains why (A) happens during pregnancy.
So, option (1) is the right answer.