High Blood Pressure of Pregnancy

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What is high blood pressure?

High blood pressure (hypertension) means that the pressure of the blood in your arteries is too high. Blood pressure is recorded as two figures. For example, 140/85 mmHg. This is said as '140 over 85'. Blood pressure is measured in millimetres of mercury (mmHg).

The first number is the systolic pressure. This is the pressure in the arteries when the heart contracts. The second number is the diastolic pressure. This is the pressure in the arteries when the heart rests between each heart beat.

• Mildly high blood pressure is 140/90 mmHg or above, but below 160/100 mmHg.
• Moderate to severe high blood pressure is 160/100 mmHg or above.

• just a high systolic pressure, for example, 170/70 mmHg.
• just a high diastolic pressure, for example, 130/104 mmHg.
• or both, for example, 170/110 mmHg.

However, any substantial rise in the blood pressure from a reading taken in early pregnancy is a concern, even if it does not get as high as the levels listed above. (You may have quite low blood pressure to start with.)

What are the different types of high blood pressure in pregnancy?

High blood pressure caused by the pregnancy
About 1 in 4 women develop high blood pressure in their first pregnancy. It is less common in further pregnancies. If you develop high blood pressure, it usually first becomes high after the 20th week of pregnancy. It usually returns to normal after the pregnancy. Some women who have high blood pressure caused by pregnancy develop pre-eclampsia (see below).

The reason why pregnancy may cause high blood pressure is not fully understood. It probably has something to do with a slight problem with the placenta (the afterbirth). This is the attachment between the baby and the mother's uterus (womb). Somehow this may affect the blood pressure in the mother.

Other causes
Some women already have high blood pressure before they become pregnant. There are various causes. See separate leaflet called 'High Blood Pressure' for details.

If you are taking medication to treat high blood pressure then, ideally, you should have this reviewed before you become pregnant. Some drugs that are used to treat high blood pressure should not be taken during pregnancy. For example, drugs called ACE inhibitors that are commonly used to treat high blood pressure can harm a developing baby and should not be taken when you are pregnant. If you are taking one of these drugs then you are likely to be switched to another drug that does not harm a developing baby.

What is pre-eclampsia?

Pre-eclampsia is a condition that develops in about 1 in 5 women who have high blood pressure during pregnancy. Pre-eclampsia not only causes high blood pressure, but it also affects other parts in the mother's body such as the kidneys, liver, and blood vessels. Pre-eclampsia causes protein to leak from the kidneys into the urine. This is why your urine is tested regularly for protein when you are pregnant. See a separate leaflet called 'Pre-eclampsia' for more details.

As a rule, the higher the blood pressure, the greater the risk to mother and baby.

• If the high blood pressure remains mild and you do not develop pre-eclampsia, then the risk is low. Most women with high blood pressure of pregnancy have just mildly raised blood pressure. However, it is important that your blood pressure and urine are checked regularly throughout the pregnancy.
• Severe high blood pressure, especially with pre-eclampsia, is serious.
  • The risks to the mother are: an increased chance of having a stroke; damage to the kidneys and liver; an increased risk of blood clots.
  • The risks to the baby are an increased chance of poor growth and stillbirth.

Pregnant women with high blood pressure are usually seen by a specialist (obstetrician). The specialist has to take into account such things as:

• How severe is the high blood pressure?
• Is there pre-eclampsia, and if so, how severe is it?
• How far on is the pregnancy?
• What are the risks to the mother and baby. This will depend on the severity of the high blood pressure (and pre-eclampsia).

If high blood pressure remains mild and pre-eclampsia does not develop
There is little risk. Regular checks to see how the pregnancy is progressing may be all that is required until the natural time of birth.

If high blood pressure becomes severe or if pre-eclampsia develops
In this situation, there is often a dilemma. If the high blood pressure is caused by the pregnancy, the only cure is to deliver the baby. This may be fine if the pregnancy is near to term. The birth can be induced, or the baby can be born by caesarean section if necessary.

However, a difficult decision may have to be made if the blood pressure or pre-eclampsia becomes severe earlier in pregnancy. Medication to lower the blood pressure may be prescribed for a while. This may allow the pregnancy to progress further before delivering the baby. If you have pre-eclampsia, magnesium sulphate may be given via a drip around the time the baby is delivered. The best time to induce the birth (or deliver by caesarean section) varies depending on the factors mentioned above.

Further help and information

APEC (Action on Pre-EClampsia)   84-88 Pinner Road, Harrow, Middlesex, HA1 4HZ
Helpline: 020 8427 4217 (Weekdays 10am - 3pm)    Web: www.apec.org.uk

© EMIS and PIP 2006   Updated: June 2006   PRODIGY Validated

The authors and editors of this article are employed to create accurate and up to date content reflecting reliable research evidence, guidance and best clinical practice. They are free from any commercial conflicts of interest. Find out more about updating.

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