Blood pressure

Measuring blood pressure at the first antenatal visit aims to identify women with chronic hypertension (high blood pressure), which may be related to existing kidney disease. After 20 weeks, high blood pressure and/or proteinuria may indicate pre-eclampsia.

24.1 Background

Healthy pregnancy is characterised by a fall in blood pressure, detectable in the first trimester, usually reaching its lowest point in the second trimester and rising to pre-conception levels towards the end of the third trimester (Lowe et al 2015). Hypertensive disorders during pregnancy include (Lowe et al 2015):

• chronic hypertension: blood pressure ≥140 mmHg systolic and/or ≥90 mm diastolic confirmed before pregnancy or before 20 completed weeks pregnancy, without a known cause (essential hypertension), associated with a secondary cause such as existing kidney disease (secondary hypertension) or associated with measurement in a healthcare setting (white coat hypertension)
• gestational hypertension: new onset hypertension (defined as a blood pressure ≥140 mmHg systolic and/ or ≥90 mm diastolic) after 20 weeks pregnancy without any maternal or fetal features of pre-eclampsia, followed by return of blood pressure to normal within 3 months after the birth
• pre-eclampsia: a multi-system disorder characterised by hypertension and involvement of one or more other organ systems and/or the fetus, with raised blood pressure after 20 weeks pregnancy commonly the first manifestation and proteinuria a common additional feature (although not required to make a clinical diagnosis) (see Chapter 26)
• superimposed pre-eclampsia: development of one or more of the systemic features of pre-eclampsia after 20 weeks pregnancy in a woman with chronic hypertension.

24.1.1 Prevalence of high blood pressure

• In Australia in 2014–15 AIHW 2016, 22% of adult women had measured high blood pressure, excluding those taking medication.
• Among Indigenous adults, 18% of women had measured high blood pressure AIHW 2016.
• A substantial number of pregnancies (0.2–5%) are complicated by pre-existing hypertension (Lowe et al 2015).
• Pre-eclampsia in the second half of pregnancy occurs in about 22% of women with chronic hypertension (Lowe et al 2015).

24.1.2 Risks associated with high blood pressure during pregnancy

Women with chronic hypertension are at greater risk of pregnancy complications, such as placental abruption, superimposed pre-eclampsia, fetal loss, preterm labour, low birth weight, perinatal death Jain 1997, Sibai 2002 and gestational diabetes (Hedderson & Ferrara 2008). Other risk factors for pre-eclampsia are discussed in Section 26.2.

24.2 Measuring blood pressure

Routine measurement of women’s blood pressure at the first antenatal visit and throughout pregnancy is recommended in the United Kingdom NICE 2008, NICE 2010 and Canada SOGC 2008. This advice reflects the importance of predicting the risk of pre-eclampsia to allow monitoring and preventive treatment. Any woman presenting with new hypertension after 20 weeks pregnancy should be assessed for signs and symptoms of pre-eclampsia (see Section 26.2).

24.2.1 Measuring blood pressure

Blood pressure should be measured as outlined below NICE 2008:

• using the woman’s right arm (Lowe et al 2015), remove tight clothing and ensure arm is relaxed and supported at heart level
• use cuff of appropriate size (eg use a large cuff if arm circumference is >33cm and a thigh cuff if it is >42cm)
• inflate cuff to 20–30 mmHg above palpated systolic blood pressure
• lower column slowly, by 2 mmHg per second or per beat
• read blood pressure to the nearest 2 mmHg
• measure diastolic blood pressure as disappearance of sounds (phase V; or IV if phase V is absent).

Women with a single diastolic blood pressure reading of 110 mmHg or more, or two consecutive readings of 90 mmHg or more at least 4 hours apart and/or significant proteinuria (1+) require increased monitoring and treatment should be considered. Women with a systolic blood pressure equal to or above 140 mmHg on two consecutive readings at least 4 hours apart require further assessment and treatment should be considered.

Automated blood pressure measuring devices

Although mercury sphygmomanometry remains the gold standard for measuring blood pressure, due to environmental and safety concerns its use is declining and automated devices are increasingly being used in the general hypertensive population (Brown et al 2011). Few studies have compared these devices with sphygmomanometry in pregnant women (Lowe et al 2015). While they may give similar mean blood pressure values to those obtained with sphygmomanometry, there is wide intra-individual error and their accuracy may be further compromised in pre-eclamptic women Gupta et al 1997, Brown et al 1998.

The potential errors of automated devices may be offset by comparing blood pressure recordings by routine mercury sphygmomanometry (Brown et al 2011). Considerations with automated devices include:

• using only devices that have been validated for use in pregnancy by the British Hypertensive Society, the Association for the Advancement of Medical Instruments or other accepted and published criteria
• maintaining some mercury sphygmomanometers to allow regular calibration of all devices
• when a pregnant woman uses an automated device for home blood pressure measurements, checking the device against mercury sphygmomanometry to ensure accuracy of readings.

White coat hypertension

White coat or “office” hypertension occurs in early pregnancy with the same frequency as it does in non-pregnant women (Brown et al 2005). A prospective study (n=241) (Brown et al 2005) found that 32% of women early in pregnancy who were given an initial diagnosis of essential hypertension had white coat hypertension. Half of these women retained this phenomenon throughout pregnancy and had good pregnancy outcomes, 40% developed (benign) gestational hypertension and also had good pregnancy outcomes and 8% developed proteinuric pre-eclampsia, which was significantly fewer than in women with confirmed essential hypertension (22%).

Women with pre-existing hypertension

Women presenting for antenatal care currently on medication for hypertension should have their medicines reviewed to ensure their safety in pregnancy.

24.3 Practice summary: blood pressure

When

At first antenatal visit.

Who

• Midwife
• GP
• obstetrician
• Aboriginal and Torres Strait Islander health worker
• multicultural health worker.

What

• Explain the risks associated with high blood pressure in pregnancy
  Discuss the importance of identifying high blood pressure early in pregnancy.
• Offer lifestyle advice
  Highlight to women who experience raised blood pressure in pregnancy the benefits of not smoking, maintaining a healthy weight, regular physical activity and a healthy diet.
• Arrange treatment or referral if required
  For women with chronic hypertension, further testing may be required to exclude white coat hypertension or kidney disease and treatment may be needed.

24.4 Resources

• British Hypertensive Society Automatic Digital Blood Pressure Devices for Clinical Use

• Lowe SA, Bowyer L, Klust K et al (2015) The SOMANZ Guidelines for the Management of Hypertensive Disorders of Pregnancy. Aust N Z J Obstet Gynaecol 55(1): 11–16.
• NICE (2010) Hypertension in Pregnancy: the Management of Hypertensive Disorders during Pregnancy. National Collaborating Centre for Women’s and Children’s Health. Commissioned by the National Institute for Health and Clinical Excellence. London: RCOG Press.
• SOGC (2008) Diagnosis, Evaluation, and Management of the Hypertensive Disorders of Pregnancy. Clinical Practice Guideline No. 206. Toronto: Society of Obstetricians and Gynaecologists of Canada.