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What is blood pressure?
It's the pressure exerted by the flow of blood through your blood vessels, from your heart to the rest of your body. In pregnancy, this pressure helps maintain a constant flow of oxygen and food though the placenta to your baby.
Your midwife will measure your blood pressure at your booking-in visit (see How it's measured, below), and all you future readings will be compared with this. A normal blood pressure range is between 95/60 and 135/85. High blood pressure depends on what's normal for you, but if the top figure (systolic) goes up by 30 or more, or the bottom (diastolic) by 20 or more, then your doctor or midwife would be concerned.
How it's measured
Your midwife will use a device known as a sphygmomanometer to take two blood pressure readings:
She will then write down the systolic measurement over the diastolic one, for example, 100/70.
Changes in pregnancy
It's common for blood pressure to alter during pregnancy. Your body is producing more blood and all your organs are having to work harder to support you and you baby. Some women have lower blood pressure than usual. This isn't dangerous, but can make you feel dizzy if you stand up too quickly.
Lots of women find their blood pressure rises in pregnancy, especially later on, but if the rise is slight and there are no other symptoms, it's not a concern. Your blood pressure readings may also vary from one visit to the next – if you've been rushing to get to an appointment, it can make it higher, while if you've had a relaxing day, it may be lower.
Are you at risk?
At your booking-in visit your midwife or doctor will try to work out if you're at risk from high blood pressure. The risk of having high blood pressure is greatest with your first baby, but if you change partner and become pregnant again, your risk is the same as a first-time mother.
You are also more at risk:
If your blood pressure becomes very high, there's a danger of your baby becoming starved of oxygen and nutrients. It could also be a sign that you've developed pre-eclampsia. If this happens, you can expect the following:
Will if affect my labour?
If your blood pressure is high towards the end of your pregnancy, then it's unlikely you'll be allowed to go beyond your due date. You may either be induced or, if your blood pressure becomes dangerously high, your baby may need to be delivered by Caesarean. You'll also be advised against having a home birth.
Will it happen again?
If you've suffered from high blood pressure in a previous pregnancy, you can expect to be closely monitored in any subsequent pregnancies although it won't necessarily happen again. Some research suggests that there is a greater risk of developing high blood pressure in later life if you have it in pregnancy, so make sure you have it checked regularly after the birth.
Real life: 'They checked me every four hours'
'Five weeks before I was due with John, I went for an antenatal appointment. The midwife found my blood pressure was up to 140/90 and there was protein in my urine. It was a Friday morning, and I was admitted to hospital that afternoon until Monday morning. They did blood tests and blood pressure tests every four hours, as well as urine checks.
'I spent the next two weekends in hospital, too. The consultant mentioned induction, but a week before my due date I went into labour and John was born with no problems. When I became pregnant for the second time, my doctor explained that I would be monitored closely but, luckily, my blood pressure stayed stable throughout.'
Tracey Bryant, is mum to John, three, and Chloe, 10 months