Both systolic and diastolic blood pressure fall as pregnancy progresses, reaching a nadir at 20 weeks gestation. Diastolic blood pressure falls to a greater extent than the systolic.

At term, maternal blood pressure returns to pre-pregnancy values.

These small changes in blood pressure, despite large changes in cardiac output and blood volume, are accounted for by large falls in systemic vascular resistance during pregnancy.

The relationship between cardiac output (CO), mean arterial blood pressure (MAP) and peripheral vascular resistance (SVR) explains this:

MAP = CO × SVR