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The definition of hypertension in pregnancy is a systolic pressure of 140 mmHg or a diastolic pressure of 90 mmHg on two consecutive occasions 4 or more hours apart. Why chose these thresholds? There is some evidence that suggests that a diastolic pressure of >95 mmHg is associated with an increased risk of stillbirth across all gestational ranges. In addition to this further studies have shown that a systolic pressure of between 140 and 150 mmHg taken together with a diastolic pressure between 90 and 95 are associated with an increase in the perinatal mortality rate. Over and above these pressures the perinatal mortality starts to rise slowly and the curve becomes steeper with the most significant increases in perinatal mortality around pressures of 170/125 mmHg. Therefore using a threshold diastolic pressure of 90 mmHg is not without some evidence. However, it is still uncertain as to whether this threshold should trigger treatment.

It is still not know whether the rise in blood pressure associated with pre-eclampsia is a protective mechanism to increase placental perfusion through already what is a partly ischaemic placenta. In this way the mother may be attempting physiologically to protect the fetus from hypoxia. Alternatively the hypertension may be a pathophysiological response to the release of "factor X" perhaps from damaged endothelium. The rise in blood pressure merely creates a vicious spiral with more endothelial damage releasing further vaso active substances. Which of these 2 explanations for the rise in blood pressure in the disease pre-eclampsia is correct is yet to be determined. Clearly if the rise in pressure is a protective effect, lowering the blood pressure may very well be detrimental to the fetus. It should be remembered that autoregulation of the maternal cerebral circulation only breaks down at blood pressures of 170/110 mmHg. The evidence that antihypertensive drugs protect the mother from morbidity is most significant at these higher levels.

In a recent meta analysis of antihypertensive therapy in pregnancy it has been suggested that lowering the blood pressure is associated with a reduction in fetal birth weight. The researchers postulated that this was due to poor placental perfusion. However, they could not rule out specific drug effects. The question "which levels of blood pressure require treatment in pregnancy?" still needs to be answered.

Once a clinician has decided to start treatment then it is clear that there are 2 main antihypertensive drugs from which to choose. The first is Methyldopa which has a long track record and is known to be safe. However, in a significant number of women, it causes severe side effects necessitating replacing it with another drug. This has led to the use of Labetalol as an alternative first line agent. This drug also has a good safety profile in pregnancy. The second line agent of choice is Nifedipine and although this drug is not licensed for use in pregnancy it has been used for many years by obstetricians with good effect.

In the acute situation Hydralazine has traditionally been the drug of choice to control high blood pressure. However recent data has suggested that oral Nifedipine may be just as effective in controlling severe hypertension although there is a theoretical problem when this drug is combined with magnesium sulphate resulting in precipitate falls of blood pressure. If this does occur there may be a consequent reduction in the placental flow causing fetal distress.

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