Blood pressure in pregnancy: what is normal and why is it checked in pregnancy

What should my blood pressure be during pregnancy?

Every time you see a doctor or midwife during pregnancy, they will undoubtedly check your blood pressure. You might be wondering what this is all about. Why is it checked so often?

Blood pressure or BP for short is a really important measure of how well your body is coping with the physiological demands of the pregnancy. It is normal in pregnancy for BP to gradually fall slightly during the first half of pregnancy and then increase gradually during the second half of pregnancy. This is because the hormones made by the placenta cause the blood vessels to open up slightly, hence the pressure within then is slightly lower. For most women, they will not notice this change during pregnancy. However, just occasionally, this BP fall can mean that you feel a bit faint if you get up too quickly for example. For the most part, as long as you feel well, a BP that is measuring lower than average during pregnancy is absolutely fine. It is a good sign that your body is adapting well to pregnancy and it does not cause the baby any problems at all.

We tend to think that the ‘average’ BP is around 120/80. However, this is probably the higher end of normal for young healthy pregnant women. Most will tend to have a BP at the beginning of pregnancy that is slightly lower than this (more often in the 100-110/ 60-65 range).  This will only likely fall a small amount by the middle of the pregnancy.

Why is my blood pressure measured?

The main reason that BP is measured to screen for pre-eclampsia, which is a disorder specific to pregnancy characterised by high BP. Pre-eclampsia can also affect the way that the kidneys and liver functions, and in the worst cases cause bleeding and even fits (a disorder call eclampsia). The risk of pre-eclampsia varies but it is probably around 2-5% of pregnancies. 

Pre-eclampsia does not usually develop until the second half of pregnancy, and the risk is higher in women who have a history of high BP before pregnancy. The midwife will therefore check your BP each time you attend.

What defines a high blood pressure?

High BP is defined as a BP of 140/90 or higher. If your BP is high then you will usually be asked to check your BP regularly at home with a home BP monitor several times per day. Some hospital use an app for you to record your BP. This is because it is common for BP to be a bit higher when it is checked by a health professional rather than what it is like when your are at home in your normal environment.

What if my blood pressure is high?

If the BP is persistently higher on the home BP monitor, then you will likely be offered treatment to keep your BP around 135/85 or lower during pregnancy. This and pre-eclampsia are discussed in another post.