PEARLS
Preeclampsia Research Laboratories
What do the blood pressure numbers mean?
Blood pressure is defined by 2 numbers: for example, 120 over 80 or 120/80.
The higher number ("systolic blood pressure") reflects the maximum pressure of the blood within the blood vessels (tubing) of the circulation. This maximum burst of pressure occurs with every beat of the heart, when blood is forced by the heart's contraction into the circulation.
The lower number (or diastolic blood pressure) is the pressure measured in between heart beats. This number reflects how full the circulation is.
There is no single normal blood pressure.
Blood pressure measurements taken during a day for any individual person may be very variable. The measurement of blood pressure is affected by the age of the person, whether they are male or female, the level of activity, stress or exercise, sleep, and most importantly, pregnancy.
Common numbers that are recorded when not pregnant are 120/80. When not pregnant, blood pressure is only called high if the readings are taken at rest, repeated several times and go above a set number of 140/90. How this reading is treated will be determined by the age of the person, whether they have other diseases and what medications they may be taking.
In pregnancy, the blood vessels are bathed in all sorts of hormones related to being pregnant. These hormones have the overall effect of lowering the blood pressure by relaxing the blood vessels.
When the circulation is narrowed, or constricted, the blood hits the walls of the blood vessels with increased pressure and the numbers measured are higher. When the blood vessels are relaxed as occurs in normal preganncy, blood pressure readings are reduced.
Therefore, it is not uncommon to have readings as low as 90/60 early in the pregnancy (often at the first visit to the doctor or midwife at 12 weeks after a missed period). There is no single normal blood pressure in pregnancy. There is a range of numbers that show whether each pregnant woman has responded to the hormones coming from her pregnancy. These numbers are affected by the age of the mother and her usual blood pressure when not pregnant.
The timing of the blood pressure measurement related to the exact number of weeks of the pregnancy is also important. Blood pressure changes between early, middle and late pregnancy. The usual pattern is for both numbers to fall from the first visit to around 24 weeks. It usually stays lower from 24 to 36 weeks and there may be a slight increase blood pressure as the time gets towards delivery. Labour itself does not usually increase blood pressure.
The other unusual feature of blood pressure in pregnancy is that the numbers do not rise as easily in response to activity or stress. This means that the mother keeps her lower pregnant blood pressure even in time of activity and stress that would normally increase blood pressure when not pregnant.
These changes in circulation in pregnancy - lower blood pressure and greater resistance to increases from the body's hormones - seem to be important to keep an adequate amount of blood flowing from the mother's circulation to the placenta which is feeding the baby. The role of the placenta in the control of blood pressure in pregnancy is a major focus of PEARLS and colleagues in preeclampsia research internationally.
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