High blood pressure in pregnancy

Blood pressure is a measure of the pressure in the large vessels (arteries) leading from the heart to the body organs. In pregnancy, this includes the uterus and placenta where the baby is growing.

A baby receives all of its food and oxygen from the mother. This means that the mother's heart has to work harder to send blood to the placenta and then to the baby.

The placenta usually has large blood vessels which make it easy for the food to get to the baby. Because these blood vessels are so big, a woman's blood pressure normally drops during the middle third of her pregnancy and returns to normal by the end of the pregnancy.

If a woman's blood pressure is too high during pregnancy this may indicate that the blood vessels in the placenta have not developed normally. This is a risk to the mother and the baby, as it can be a sign of a serious complication known as pre-eclampsia.

How blood pressure is measured

Blood pressure is shown as 2 numbers:

  • The top number (systolic) is the highest pressure in the arteries when the heart pumps blood.
  • The bottom number (diastolic) is the lowest pressure when the heart rests between heartbeats.

Blood pressure is normally written as the top number over the bottom number, such as 110/70. High blood pressure (hypertension) is diagnosed if the top number is more than 140 or if the bottom number is more than 90. This would be written as 140/90.

Types of high blood pressure in pregnancy

There are two main types of high blood pressure (hypertension) in pregnancy.

  1. High blood pressure that a woman already has before pregnancy or in the first 20 weeks is called pre-existing hypertension or chronic hypertension  High blood pressure before 20 weeks of pregnancy is not caused by pregnancy because the placenta is not fully developed.
  2. High blood pressure that a woman develops after 20 weeks of pregnancy is called gestational hypertension. Some of these women will just have high blood pressure and no other problems, but some will develop a condition called pre-eclampsia, where other organs in the body are affected in addition to having high blood pressure. 

Women with pre-existing hypertension can also develop pre-eclampsia and because they already have high blood pressure they need to be watched closely.

How is high blood pressure diagnosed?

High blood pressure usually does not cause symptoms until the blood pressure is very high, so it is usually found during a prenatal visit when you have your blood pressure checked.

At each prenatal visit, your midwife or LMC will check your blood pressure and test your urine for protein. An increase in these are warning signs of pre-eclampsia. A sudden increase in blood pressure is often the first sign of a problem.

If you have any of the following symptoms contact your midwife or lead maternity carer straight away. These could be warning signs of pre-eclampsia:
  • ongoing, persistent or severe headache
  • changes in eyesight such as seeing spots, flashing lights or floaters, blurry vision
  • pain in your upper belly, tummy area or shoulder
  • sudden and new swelling in your face, hands, and eyes (some feet and ankle swelling is normal during pregnancy)
  • sudden weight gain (more than 1 kg in a week, or more than 3 kg in a month)
  • vomiting later in your pregnancy (not the morning sickness of early pregnancy)
  • difficulty breathing.

Why is high blood pressure during pregnancy a problem?

If your blood pressure remains mildly to moderately raised and you do not develop pre-eclampsia then the risk of pregnancy complications is low. However, it is important that your blood pressure and urine should be checked regularly throughout your pregnancy.

Pre-eclampsia is a condition that affects blood vessels, so every organ in a woman's body can be affected. The kidneys are most commonly affected, but the liver, the brain, the blood clotting system and even the placenta are also often affected. Once complications start to develop in these organs delivery is usually required.

Pre-eclampsia always goes away after the baby is delivered, but not immediately and the mother may get worse for 2 or 3 days after delivery until she starts to get better.

Read more about pre-eclampsia.

How is high blood pressure during pregnancy treated?

The type of treatment will depend on two things, how bad your blood pressure is and if you have developed pre-eclampsia.

If you only have high blood pressure and NOT pre-eclampsia then usually your blood pressure can be controlled and you can have a normal pregnancy.

If pre-eclampsia does develop, you will need much closer supervision and will probably need to have your baby early. The treatment and monitoring is aimed to keep the mother and baby safe while the baby grows and develops as much as possible. If your baby does need to be delivered early there are treatments available to decrease the complications of being born prematurely. Read more about treatment and monitoring for pre-eclampsia.

Self care – what can you do to look after yourself?

To help control your blood pressure and have a healthy pregnancy:

  • Be sure to attend all your antenatal checkups so your doctor or midwife can check your blood pressure and urine protein levels. 
  • Know the signs and symptoms of pre-eclampsia and contact your midwife or lead maternity carer straight away if you have these.
  • Don't smoke. This is one of the best things you can do to lower your blood pressure and to help your baby be healthy. If you need help to quit smoking, talk to your doctor.
  • Gain an amount of weight that is healthy for you. Your doctor or midwife can help you set a pregnancy weight goal.
  • Get regular gentle exercise during pregnancy. Walking or swimming several times a week can help lower blood pressure. And it's good for you and your baby. 
  • Try to keep your stress level low. This may be hard to do, especially if you continue to work, have young children, or have a hectic schedule. But try to find some time to relax.

Learn more

Gestational diabetes and high blood pressure ADHB, NZ
Pregnancy and High Blood Pressure Patient Info, UK
High blood pressure in pregnancy Nice.org.uk

References

The SOMANZ Guideline for the Management of Hypertensive Disorders of Pregnancy Society of Obstetric Medicine of Australia and New Zealand (SOMANZ) July 2014
Hypertension in pregnancy National Institute for Health and Care Excellence (NICE) July 2013

Credits: Health Navigator Editorial Team. Reviewed By: Jeremy Tuohy, University of Auckland (7 October 2016)