Your blood pressure is determined by the balance between the amount of blood going into the vascular system and the amount going out. The first number is the systolic blood pressure, referring to the maximum force caused by the heart's contraction, and the second number is the diastolic blood pressure, meaning the pressure during the relaxation phase between heartbeats. When your blood pressure rises above a certain point, it's considered abnormal and is called hypertension, or high blood pressure. The more elevated your blood pressure, the greater the risk to you and to your pregnancy. Low levels of elevated blood pressure can, in time, result in damage to the mother's heart, kidneys, and blood vessels.
More severe elevations can lead to heart failure or even stroke.
During pregnancy, blood pressure levels normally decrease. In a pre-existing hypertension condition, however, your blood pressure may or may not decrease. Before trying to conceive, you should definitely seek medical counseling to discuss the severity of your hypertension and to evaluate its possible effects on your planned pregnancy. Your blood pressure should be brought under careful control, and medications you routinely take may need to be changed to those that will be safe during pregnancy. Your doctor may advise testing of your kidney or heart function.