Blood pressure is vital for the body's circulatory system—the heart, arteries, and veins—to function. It is created in part by the steady beating of the heart. Each time the heart contracts, or squeezes, it pumps blood into the arteries. The arteries carry the blood to the body's organs. The veins return it to the heart.

Blood pressure changes from person to person. It changes often during the day. It can increase if you are excited or if you exercise. Most often, it decreases when you are resting. These short-term changes in blood pressure are normal. It is only when a person's blood pressure stays high for some time that it may signal a problem.

In most pregnant women, readings less than 120/80 millimeters of mercury (mmHg) are normal. If you are pregnant and your systolic pressure is 140 millimeters of mercury (mmHg) or the diastolic pressure is 90 millimeters of mercury (mmHg) on two separate measurements at least four hours apart, it is too high. Your normal blood pressure can be an average of a number of readings taken at rest.