Doctors measure blood pressure with a sphygmomanometer, which has a cuff that's wrapped around the upper arm and pumped up to create pressure. When the cuff is inflated, it compresses a large artery in the arm, stopping the blood flow for a moment. Blood pressure is measured as air is gradually let out of the cuff, which allows blood to flow through the artery again.
The doctor or nurse will also put a stethoscope over an artery to hear the first pulse as the blood flows through – this is the systolic pressure (or the pressure at the peak of each heartbeat). The diastolic pressure (the pressure when the heart is resting between beats) is noted when the sounds disappear.
When a blood pressure reading is taken, the higher number represents the systolic pressure and the lower number represents the diastolic pressure. For example: 120/80 (120 over 80) means that the systolic pressure is 120 and the diastolic pressure is 80.
Blood pressure also can be measured by automated devices, which are good for screening, but a manual blood pressure is more accurate.
As kids grow, their blood pressure increases from a systolic pressure of about 70-90 in an infant to adult values (90-120) in a teenager. Among young kids, the "normal" range will depend on gender, age and height; your doctor will be able to compare your child's blood pressure with national norms.
In kids, high blood pressure is defined as a blood pressure greater than the 95th percentile for their age, height and gender. In other words, 95% of kids of the same age, height and gender will have blood pressure below this number.
It's not unusual for a first blood pressure reading to be high because a child is nervous, so the doctor will likely take three readings and use an average of the three to determine whether your child has high blood pressure or is at risk for developing it.
Some doctors use a test called ambulatory blood pressure monitoring in which a child wears a blood pressure cuff all day. Some consider it more accurate than blood pressure tests in the doctor's office because the child is less likely to be affected by any stress from the doctor visit.