Blood pressure falls in the second trimester and then rises in the third trimester.
Blood pressure measurement is taken whilst the patient is in a semi-recumbent position. If the patient is lying then supine hypotension may result as the gravid uterus can impair venous return via the inferior vena cava. In about 5% of pregnant women a standard cuff (12 x 23 cm) will not encompass the arm of hypertensive pregnant mothers and therefore a large cuff should be used in these cases. The Korotkoff IV phase is recommended to be used as the measure of diastolic blood pressure in women by the World Health Organisation. The argument for this is because, in some pregnant women, the Korotkoff V phase is said to be heard at zero cuff pressure. However arecent studies have shown that this assertion is unfounded and suggest that Korotkoff V should be measured as the diastolic pressure in pregnancy (1,2). The use of Korotkoff V for recording diastolic pressure is recommended by the concensus report from the United States National Heart, Lung and Blood Institute.
Summary points about measuring blood pressure in pregnancy
- use of a mercury sphygmomanometers are preferable to automated blood pressure monitors
- if automated devices are used they should be calibrated, and checked regularly, against a mercury sphygmomanometer
- use an appropriate size cuff
- for arm circumference of up to 33cm – standard size (13x23 cm)
- for arm circumference between 33 and 41 cm – large size (33x15 cm)
- for an arm circumference of 41 cm or more – a thigh cuff (18x36 cm)
- using a too large cuff than too small a cuff minimize errors (3)
- woman should be seated or lying at 45° angle, with arm at level of the heart, blood pressure should not be measured when the women is lying on her side since this will give lower readings (3)
- blood pressure is needed to the nearest 2 mm Hg
- use phase V Korotkoff sound (sound disappearance) to measure diastolic blood pressure
- this is subject to less intra-observer and inter-observer variation than Korotkoff IV (muffling of heart sounds) and also seem to correlate best with intra-arterial pressure in pregnancy
- in around 15 % of pregnant women, diastolic pressure reduces to zero before the last sound is heard. In these patients both phase IV and phase V readings should be indicated e.g. - 148/84/0 mmHg (3)
Ambulatory blood pressure monitoring (ABPM):
- useful for assessing blood pressure early in pregnancy (<20 weeks)
- one third of these patients will have “white coat” or “office” hypertension
- 50% of these women will not require treatment for hypertension during pregnancy while the other half will develop true hypertension
- ABPM is less beneficial during second half for detection of white coat hypertension
- ABPM is also useful to predict the risk of developing hypertension later in pregnancy (although sensitivity and specificity is low) (5)
Hypertension in pregnancy and pre-eclampsia are discussed in the menu items below.