At her 16-week checkup, a patient's blood pressure is slightly decreased from her pre-pregnancy level. You evaluate this change based on which of the following statements concerning blood pressure during pregnancy?

a) Normally, blood pressure increases steadily throughout pregnancy.
b) Blood pressure remains stable until decreasing the day of the delivery.
c) Blood pressure progressively decreases throughout the entire pregnancy.
d) A decrease in the second trimester may occur because of placental growth.