CLASSIFICATION OF HYPERTENSIVE DISORDERS OF PREGNANCY
There are three types of hypertensive disorders:
- chronic hypertension
- gestational hypertension
1. Chronic hypertension
Chronic hypertension complicates 3–5% of pregnancies although this figure may rise, with the trend for women to postpone childbirth into their 30s and 40s. The diagnosis of chronic hypertension is based on a known history of hypertension pre-pregnancy or an elevated blood pressure ⩾ 140/90 mm Hg before 20 weeks gestation. However, there are several caveats to this diagnosis. Undiagnosed hypertensive women may appear normotensive in early pregnancy because of the normal fall in blood pressure, commencing in the first trimester. This may mask the pre-existing hypertension and when hypertension is recorded later in the pregnancy it may be interpreted as gestational. Sometimes the diagnosis is only made several months post partum, when the blood pressure fails to normalise as would be expected with gestational hypertension. Furthermore, pre-eclampsia can rarely present before 20 weeks gestation and may be misinterpreted as chronic hypertension.
The presence of mild pre-existing hypertension approximately doubles the risk of pre-eclampsia but also increases the risk of placental abruption and growth restriction in the fetus. In general, when blood pressure is controlled, such women do well and have outcomes not dissimilar to normal women. However, when chronic hypertension is severe (a diastolic blood pressure > 110 mm Hg before 20 weeks gestation) the risk of pre-eclampsia is as high as 46% with resultant raised maternal and fetal risks.
2. Gestational hypertension
Hypertension occurring in the second half of pregnancy in a previously normotensive woman, without significant proteinuria or other features of pre-eclampsia, is termed gestational or pregnancy induced hypertension. It complicates 6–7% of pregnancies and resolves post partum. The risk of superimposed pre-eclampsia is 15–26%, but this risk is influenced by the gestation at which the hypertension develops. When gestational hypertension is diagnosed after 36 weeks of pregnancy, the risk falls to 10%. With gestational hypertension, blood pressure usually normalises by six weeks post partum.
3. Pre-eclampsia and eclampsia
Pre-eclampsia usually occurs after 20 weeks gestation and is a multi-system disorder. It was classically defined as a triad of hypertension, oedema, and proteinuria, but a more modern definition of pre-eclampsia concentrates on a gestational elevation of blood pressure together with > 0.3 g proteinuria per 24 hours. Oedema is no longer included because of the lack of specificity. Pre-eclampsia may also manifest, with few maternal symptoms and signs, as isolated intrauterine growth restriction (IUGR). Eclampsia is defined as the occurrence of a grand mal seizure in association with pre-eclampsia, although it may be the first presentation of the condition.
The incidence of pre-eclampsia is very much influenced by the presence of existing hypertension, although other risk factors are recognised (table 11). Overall pre-eclampsia complicates 5–6% of pregnancies, but this figure increases to up to 25% in women with pre-existing hypertension. Eclampsia complicates 1–2% of pre-eclamptic pregnancies in the UK. An estimated 50 000 women die annually from pre-eclampsia worldwide and morbidity includes placental abruption, intra-abdominal haemorrhage, cardiac failure, and multi-organ failure. In the last confidential enquiry into maternal deaths there were 15 confirmed deaths from pre-eclampsia or eclampsia, the majority as a result of intracerebral haemorrhage. The risks to the fetus from pre-eclampsia include growth restriction secondary to placental insufficiency, and premature delivery. Indeed, pre-eclampsia is one of the most common causes of prematurity (accounting for 25% of all infants with very low birth weight, < 1500 g).