Hypertension is the most common medical problem encountered in pregnancy and remains an important cause of maternal, and fetal, morbidity and mortality. It complicates up to 15% of pregnancies and accounts for approximately a quarter of all antenatal admissions. The hypertensive disorders of pregnancy cover a spectrum of conditions, of which pre-eclampsia poses the greatest potential risk and remains one of the most common causes of maternal death in the UK.

NORMAL PHYSIOLOGICAL CHANGE IN BLOOD PRESSURE DURING PREGNANCY

Early in the first trimester there is a fall in blood pressure caused by active vasodilatation, achieved through the action of local mediators such as prostacyclin and nitric oxide. This reduction in blood pressure primarily affects the diastolic pressure and a drop of 10 mm Hg is usual by 13–20 weeks gestation.¹ Blood pressure continues to fall until 22–24 weeks when a nadir is reached. After this, there is a gradual increase in blood pressure until term when pre-pregnancy levels are attained. Immediately after delivery blood pressure usually falls, then increases over the first five postnatal days.^w1 Even women whose blood pressure was normal throughout pregnancy may experience transient hypertension in the early post partum period, perhaps reflecting a degree of vasomotor instability.

DEFINITION OF HYPERTENSION IN PREGNANCY AND BLOOD PRESSURE MEASUREMENT

Hypertension in pregnancy is diagnosed either from an absolute rise in blood pressure or from a relative rise above measurements obtained at booking. The convention for the absolute value is a systolic > 140 mm Hg or a diastolic > 90 mm Hg. However, it should be recognised that blood pressure is gestation related. A diastolic blood pressure of 90 mm Hg is 3 standard deviations (SD) above the mean for mid pregnancy, 2 SD at 34 weeks, and 1.5 SD at term.^w2 The definition for a relative rise in blood pressure incorporates either a rise in systolic pressure of > 30 mm …