Recent classifications from American (Anonymous, 2000, 2002), Canadian (Helewa et al., 1997) and
Australasian (Brown et al., 2000) representative bodies have defined gestational hypertension as a blood pressure >140/90mmHg after 20weeks gestation. In a general pregnant population, blood pressure greater than 140/90 mmHg is more than two standard deviations above the mean between 20 and 34 weeks gestation, and approximately two standard deviations above the mean from 35 weeks to term (Stone et al., 1995). There are minor differences in the criteria for gestational hypertension between these consensus statements. The Canadian's elected to use only a diastolic blood pressure >90 mmHg in the definition, requiring confirmation on at least two occasions 4 h apart. The Australasian definition specifies a systolic blood pressure greater or equal to 140mmHg and/or diastolic blood pressure of at least 90 mmHg, taken repeatedly over several hours and an absence of any features of multisystem disease. In the USA, the National Institutes of Health working group report on blood pressure in pregnancy, endorsed by the American College of Obstetricians and Gynecologists (ACOG), has defined gestational hypertension as a systolic blood pressure greater than 140mm Hg or diastolic blood pressure greater than 90 mmHg after 20 weeks gestation, without reference to repeated recordings. All classification systems include women in whom the blood pressure rise is only present intrapartum, and in the USA this may comprise a substantial proportion of all pre-eclamptics (Zhang et al., 2001). As gestational hypertension may progress to pre-eclampsia (Barton et al., 2001; Saudan et al., 1998), it is recognized that the final diagnosis of gestational hypertension can only be made postpartum (Anonymous, 2000). Resolution of the high blood pressure by 3 months postpartum is required in order to confirm the woman does not have chronic hypertension.
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