Blood pressure is the pressure with which blood moves through your arteries. These are the blood vessels that carry blood from your heart to all parts of your body. 
Doctors and nurses measure your blood pressure by putting an inflatable cuff around your upper arm. The measurement is usually taken automatically by a machine.
The result is given as two numbers.
The first (top) number is your systolic blood pressure. That's the pressure when your heart squeezes blood out during beats.
The second (bottom) number is your diastolic blood pressure. That's the pressure when your heart relaxes between beats.
Blood pressure varies between people. But a typical normal reading for a young pregnant woman is a top number (systolic pressure) of 110 to 120 millimetres of mercury (mm Hg for short) and a bottom number (diastolic pressure) of 70 to 80 mm Hg.
This would be written down as, for example, 120/80 mm Hg. Your doctor or nurse will say your blood pressure is "120 over 80". 
If your blood pressure is lower than normal, it's not usually a problem. But if it is higher than normal, it can damage your arteries. This puts you at risk for heart disease, stroke, and kidney disease.
Your blood pressure is high if the top reading (your systolic pressure) is higher than 140 and the bottom reading (diastolic pressure) is higher than 90.  In other words, your blood pressure is higher than 140 over 90.
During the first half of your pregnancy, your blood pressure will probably go lower than normal. This is because your hormones make your arteries open wider. During the second half of your pregnancy, your blood pressure will slowly rise back to normal.
If your blood pressure is higher than normal in the first half of pregnancy, it probably means you have had a blood pressure problem since before you got pregnant. Maybe you just didn't know about it.
If you get high blood pressurein the second half of pregnancy, it means one of three things.
You had high blood pressure before you got pregnant (and maybe you didn't know it).
Your blood pressure became high for the very first time in pregnancy. This means you may get high blood pressure later in your life.
You have pre- eclampsia. In this case, you will also have protein in your urine. You may have other signs and symptoms too.
If your doctor thinks you have just high blood pressure (but not pre-eclampsia), he or she will watch you carefully until after your baby is born. This is because you could still get pre-eclampsia at any time.