Blood Pressure in Pregnancy

What is blood pressure?

Blood pressure is the pressure of your blood on the walls of your arteries as your heart pumps it around your body. It’s a vital part of how your heart and circulation works. Once the blood leaves your heart, it continues to move through the rest of your body via your arteries under two forces:

  1. The blood from your next heartbeat pushes it away from your heart, and
  2. The muscles in the walls of your arteries contract and relax, moving your blood in the right direction.

Arteries play an active role in moving blood around your body. Therefore, they have a thick muscle layer. The muscular artery walls are elastic and can stretch to accommodate higher volumes of blood; for instance, when your heart beats faster, or you are exercising. This elasticity regulates how much pressure the artery walls are under while doing their job. Veins, on the other hand, are almost passive bystanders in returning blood to your heart. They don’t need to pump it back – it’s more like a shuffle. It is for this reason that they have valves, rather than thick muscle walls. The valves close and let blood go one way – toward your heart – but not the other (when they work correctly).

What do the numbers mean?

We record blood pressure as two numbers – a top and a bottom (120/70). The top number is the systolic reading – a fancy name for the stress on your artery walls when your heart is contracting (or squeezing the blood out). The normal range for this is between 90 and 120. The bottom number is the diastolic reading. This reading is the pressure on your artery walls when your heart is relaxing (or refilling) and is normal if your reading is between 60 and 80. Low blood pressure (hypotension – readings lower than these numbers), in otherwise healthy people, is not associated with any long-term health risks. When you are hypotensive, you may feel dizzy or lightheaded, unsteady on your feet, or faint. It can happen when we have been standing for a long time, or we get up too quickly.

Any changes in position need to be detected by our body, and our blood diverted accordingly. If you are prone to low blood pressure, take your time getting up and take a seat now and then. It is more likely to happen when we are hot because our body diverts more blood to your skin to try and cool you down. While uncomfortable, it is easily treatable – a big drink of water and taking a moment to adjust should make you feel better quickly. If this does not resolve your situation, please see your doctor.

What is normal?

Your blood pressure is high if it is persistently above the normal range. If your systolic is between 120-140, or your diastolic is between 80-90, it is still in the normal range. However, it may indicate that problems are just around the corner, and extra surveillance may be necessary.

Measurements over 140 systolic and/or 90 diastolic are abnormal and require further review. If this high blood pressure does not resolve and becomes your usual reading, the condition is termed chronic. The medical name for high blood pressure over a long period is chronic (or essential) hypertension.

Chronic hypertension is one of the main risk factors for heart disease. Your risk for these increases as you age, but it is now not uncommon to find young adults with hypertension. Please get your blood pressure checked regularly. If it’s persistently high, it needs management to minimise your risk of other health complications. Uncontrolled hypertension can lead to a heart attack, stroke, or kidney damage.

How will my blood pressure change in pregnancy?

Your blood pressure is influenced at different stages of your pregnancy by how your body adjusts to its new demands. The hormone progesterone, which is responsible for maintaining your pregnancy, also causes the walls of your blood vessels to relax. This relaxation results in a drop in your blood pressure, sometimes to the point where you feel faint. Your blood pressure will be at its lowest mid-pregnancy, however as you progress, your body starts to make extra blood.

Blood pressure will start to rise again by about 24 weeks, with most women returning to normal in the last few weeks before baby’s birth. Hypertension is another matter and can lead to serious consequences for you and your baby during pregnancy. High blood pressure has different names and causes, depending on how many weeks pregnant you are when you develop it.

If you are under 20 weeks pregnant, your hypertension will likely be chronic. This term means you probably had high blood pressure before you became pregnant and just didn’t know it. After 20 weeks, high blood pressure in pregnancy is called gestational hypertension, or pregnancy-induced hypertension (PIH).

Some PIH is not a problem and can be managed with observation and/or medication. However, it does have the potential to evolve into pre-eclampsia or eclampsia – an abnormal condition and potentially severe complication of pregnancy.

Checking your blood pressure

Your midwife or doctor will check your blood pressure at every antenatal appointment as part of your routine pregnancy screening (20). This check is an integral part of your antenatal care. Hypertension does not always make you feel unwell, and you may show no signs of pre-eclampsia until it is well advanced.

This surveillance is just one of the many reasons for attending your recommended antenatal appointments. During a prenatal appointment, if your doctor or midwife finds a high reading, she should suggest a further review. This review may involve a few actions, including (21):

  • Making sure your result is not due to exertion or stress. You may need to wait and chat about other things, then retest a little while later.
  • Monitoring your baby to make sure your baby is coping 
  • A 3-4 hour stay at the hospital to monitor your blood pressure hourly
  • A urine test to check for protein in your urine
  • Blood tests to screen for pre-eclampsia
  • A doctor review

What does high blood pressure mean for me?

You have hypertension if:

  • Your blood pressure is 140 systolic and/or 90 diastolic or higher for two readings, four hours apart, or
  • You have a one-off diastolic pressure of 110 or more. This reading is more important than the systolic in telling us about your health. If this figure alone is high, you have high blood pressure.

If you are hypertensive, it is essential to discuss the signs and symptoms of pre-eclampsia with your care provider. If you are in this situation and feel unwell or odd for any reason, please contact your caregiver straight away. It is better to get you in for a check-up and find that all is well than becoming very sick because you don’t want to bother them.

Please click here to read more information about hypertension during pregnancy.

Dr Janelle McAlpine (PhD), Clinical Midwife
Photo by Emilia Ungur used under license from Shutterstock.com