During pregnancy, the caregiver will normally place the blood pressure cuff on your right arm while you are sitting comfortably with your feet supported. Some caregivers will position the woman differently, or use both arms to measure (doing the blood pressure twice to compare). Whatever method they use, it is important that they take your blood pressure in a consistent manner, so that relevant comparisons can be made as your pregnancy progresses. This is because the diagnosis of high blood pressure during pregnancy is often determined by comparing blood pressure readings taken during early pregnancy with subsequent readings taken during late pregnancy.
The caregiver uses a stethoscope on your arm to listen to the 'beat' of the blood flowing through the blood vessels. The pressure device is called a 'sphygmomanometer' and is pumped up past the expected highest level (normally about 140-150 mmHg), this can feel quite tight on your arm. The air in the cuff around your arm is then slowly let out. Your caregiver takes note of the first reading when the 'beat' is first heard through the stethoscope. This is called the 'systolic' reading (or the top number, for example 110). The beat continues to be listened to until the sound disappears, this is noted as the second reading and is recorded as the 'diastolic' reading (or the bottom number, for example 60).
A pregnant woman may have a normal blood pressure as low as 90/50 or as high as 135/80, with an average of about 110/70. The pressure itself is not as much of an issue as how much it rises over the course of the pregnancy. Men tend to have blood pressures ranging from 110/70 to about 140/80 with an average of about 120/80.