Normal pregnancy blood pressure
Your blood pressure is taken at every pregnancy visit as well as intermittently during labour and occasionally after the birth, depending on your health. When a caregiver takes a person's blood pressure, they measure the pressure exerted by the blood against the walls of the blood vessels in the arteries.. The reading is determined by many things including:
The pumping action of the heart,
or how hard the heart pumps the blood. This is the highest reading (eg. 120 mmHg if your blood pressure is 120/80) and is known as the systolic measurement, which fluctuates with stress, emotion, anxiety and exercise, possibly increasing this reading.
The lower reading is the pressure on the blood vessels between the heart pumps or when the heart is at rest (eg. 80mmHg if your blood pressure is 120/80), known as the diastolic measurement. The diastolic pressure is the constant, lower pressure on the blood vessels and generally the measurement your caregiver focuses on during pregnancy.
Resistance to the flow of blood in the smaller arteries (or arterioles).
The smaller arteries in the body can control blood pressure through tightening and relaxing. If they tighten, they increase their resistance and do not allow blood to flow as freely, increasing blood pressure and decreasing blood flow to the smaller vessels in the body (as well as possibly the placenta and baby during pregnancy). This is one physical side effect of very high blood pressure during pregnancy called pre-eclampsia. A tightening of the arterioles is also caused by smoking. You can read more in smoking and lifestyle changes during pregnancy.
Elasticity of the walls of the large arteries.
During pregnancy, the blood vessels relax under the influence of the hormone progesterone, increasing their elasticity and lowering blood pressure. Therefore it is normal for blood pressure to decrease slightly during the first 12 weeks, drop significantly from about 12 to 28 weeks, being the most common cause for a pregnant woman feeling faint or dizzy.
The thickness or viscosity of the blood.
During pregnancy the fluid component of the blood (called plasma) increases in volume by up to 50%. This dilutes the blood cells floating in the fluid, decreasing the thickness of the blood, contributing to a lower blood pressure, especially during the first 32 weeks of pregnancy.
The amount of blood inside the blood vessels
or blood volume increases by up to 50% during the first 32 weeks of pregnancy, creating a dramatic pressure change that begins to outweigh the relaxation effect of the progesterone hormone and the dilution of the blood as the pregnancy progresses. As the pressure increases, the woman's blood pressure begins to creep back up (usually during the last 12 weeks of pregnancy), often back to levels she experienced before her pregnancy started, or even slightly higher.
A person's blood pressure reading can fluctuate every minute of the day. Your average reading is very individual to you and will depend on many factors, such as:
- Your sex. Men generally have higher blood pressures than women.
- Your age. The older you are, the higher your blood pressure will tend to be. A newborn baby has a blood pressure of about 80/40, a teenager about 100/60, an adult about 110/60 to 130/80 and an elderly person about 140/85.
- Your build and weight. Increased weight tends to increase your blood pressure.
- Your fitness. The fitter you are, the lower your blood pressure will tend to be.
- The time of day. Blood pressure is normally lower when you first wake, and higher at the end of the day.
- Your position. During middle to late pregnancy, lying flat on your back will often give a higher reading than if you are sitting up (because of the pressure of the uterus on the aorta and vena cava blood vessels know as aortocaval compression). Lying on your left side generally gives the lowest reading.
- Which arm is used. The blood pressure reading can be different if taken on the right or left arm.
- The appropriate sized blood pressure cuff. If your arm is larger, your caregiver will need to use a larger cuff to get an accurate reading (the part of the machine that is wrapped around your arm).
- Your physical activity. Blood pressure is normally lower when you are resting.
- Your emotional state. Feeling stressed, anxious, nervous or excited will increase your blood pressure (when seeing a doctor it is called 'white coat syndrome'!.
- The stage of pregnancy. Blood pressure is generally slightly lower during the first 12 weeks, dropping significantly from 12 to 28 weeks and rising to pre-pregnancy levels (or slightly higher) after about 28 weeks.
- Your family history. Hereditary factors from your parents and relatives can make you prone to having high (or low blood) pressure.
- Whether you smoke. Smoking increases your blood pressure.
Normal pregnancy blood pressure
During pregnancy, the caregiver places the blood pressure cuff on your right arm while you are sitting comfortably with your feet supported. Some caregivers position women 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 later in the pregnancy.
The caregiver uses a stethoscope on your arm to listen to the beat of the blood flowing through the 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 listens for the first beat heard through the stethoscope, which is the systolic reading (or the top number, for example 110). The beat continues to be listened to until the sound disappears, which is noted as the second reading, recorded as the diastolic or 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.
You can read about the causes of high blood pressure in blood pressure during pregnancy or about very high pregnancy blood pressure, called pre-eclampsia.
Updated October 2008
Last revised: Saturday, 10 November 2012
This article contains general information only and is not intended to replace advice from a qualified health professional.