Hypertensive disorders complicate 5-10% of pregnancies and contribute greatly to maternal and fetal morbidity and mortality throughout the world [1–3]. Population-based Australian studies undertaken in Victoria [4] and South Australia [5] have shown a prevalence of 9.8% and 9.2% respectively, however, Australian Indigenous women have been reported as having 66% greater risk for pregnancy-induced hypertension when compared with non-Indigenous women [6]. Australian Aboriginal and Torres Strait Islander women are also at comparatively higher risk for developing chronic hypertension and cardiovascular disease at a younger age than other Australian women [7, 8]. Poor cardiovascular health prior to pregnancy modulates maternal preeclamptic syndrome so that frequency and severity of hypertensive disease in pregnancy are increased [9]. Women with chronic hypertension may have uneventful pregnancies when their hypertension remains mild to moderate, however, overall outcomes are poorer than for normotensive women and they are at at greater risk of developing preeclampsia (≥20%) [10]. Preeclampsia occurs in about 3-5% of pregnancies [11] and is the hypertensive condition whether it occurs alone or superimposed on chronic hypertension most often associated with maternal and fetal complications including fatalities [1]. The disorder is an early marker not only for future maternal cardiovascular and metabolic disease, [1, 11, 12] but also confers long-term risk for cardiovascular disease among infants [2]. Hypertension during pregnancy has been identified as a predictor of poor birth outcomes in Aboriginal and Torres Strait Islander women in north Queensland [13]. However, it remains unclear why this group of women have an increased risk for hypertension in pregnancy.

Pre-pregnancy overweight and obesity have been identified as risk factors for hypertensive disorders of pregnancy [14, 15]. In the United States, obesity is the leading risk factor for preeclampsia; it confers a three-fold increased risk and is present in 30% of cases [16]. Now a major epidemic in developed countries including Australia, obesity is impacting on the health and well-being of Indigenous Australians. With the change from traditional carbohydrate based diets to energy dense high fat diets, combined with lower levels of physical activity [17], rates of obesity among Aboriginal and Torres Strait Islander women of childbearing age have escalated [18]. We examined pre-pregnancy factors that may predict any hypertension in pregnancy in a cohort of Aboriginal and Torres Strait Islander women in north Queensland. We hypothesised that abnormal pre-pregnancy blood pressure levels and maternal adiposity would increase the risk of hypertensive disorders during pregnancy.

Twenty-two of the 220 women who gave birth in the 8–10 years after their health check had hypertension during their pregnancy. Seven of these women (3.2%) were diagnosed with moderate (n = 6) or severe (n = 1) preeclampsia, a rate equivalent to that found among other population groups [11]. The strongest pre-pregnancy predictors of hypertension during pregnancy in this cohort of Aboriginal and Torres Strait Islander women were central adiposity (assessed by waist circumference), obesity (assessed by BMI) and an elevated blood pressure recording at the WPHC cross-sectional survey. By studying a sample of the Indigenous populations of Australia, which have some of the highest prevalence of hypertensive pregnancy, we have emphasised pre-pregnancy obesity and hypertension as independent contributors to this disease. Both of these risk factors are potentially amenable to intervention before pregnancy.

Our results add to previous Australian research that has identified elevated pre-pregnancy BMI as an important factor in the development of serious hypertensive disorders of pregnancy [27]. A systematic review of international controlled studies of risk factors for preeclampsia at antenatal booking showed that raised BMI before pregnancy (relative risk 2.47; 1.66-3.67) is one of the few amenable to intervention [28]. Other factors known to increase risk that cannot be altered prior to pregnancy include previous history or family history of preeclampsia, multiple pregnancy, nulliparity, and maternal age among multiparous women. Our study highlights the need to focus health promotion efforts on encouraging healthy body weight among Aboriginal and Torres Strait Islander women of childbearing age.

Elevated pre-pregnancy blood pressure in this study was an additional predictor of a hypertensive disorder of pregnancy. Hypertension among Indigenous women and men aged 25–50 years has previously been shown to be highly prevalent in communities included in the study [12]. Pre-pregnancy blood pressure assessment in this high risk population is important because undiagnosed chronic hypertension can be masked in early pregnancy due to physiological changes that may cause initial decreases in blood pressure, then misdiagnosed later in the pregnancy as a gestational disorder when abnormal readings re-surface [1]. One of the best predictors of chronic hypertension for Aboriginal and Torres Strait Islander Australians is overweight and obesity established by elevated waist circumference [17].

Aside from pre-pregnancy adiposity and raised blood pressure, two additional risk factors associated with poor metabolic health, elevated albumin:creatinine and the γ -glutamyl transferase, increased the risk for hypertension during pregnancy among the cohort. Raised albumin:creatine ratio as a marker of kidney damage and raised γ -glytamyl transferase as a marker of non-alcoholic fatty liver disease or risky alcohol use, provide a general picture of poor health in women of childbearing age. Among women in the cohort free of diabetes at baseline, presence of metabolic syndrome as defined by the International Diabetes Federation criteria, showed over a three-fold risk for hypertension during pregnancy. From a population health perspective, health risks among this cohort are exacerbated by high rates of smoking and risky alcohol use and low levels of physical activity. The extent of health challenges faced by these young women, and our improved understanding of how ‘fetal programming’ predicts the immediate and long term health of the next generation, call for interventions well before pregnancy begins in this population.

Encouraging weight loss during pregnancy is not a suitable strategy; weight loss outside of pregnancy requires lowering caloric intake or raising energy expended. Programs that incorporate a combination of physical activity and nutrition education have been shown to reduce body weight [29] and blood pressure [30, 31]. In this study, less than 20% of the women reported undertaking adequate physical activity in the week before their baseline health check. It has been shown that when women exercise between 3 months and 1 year before or during pregnancy, the risk of preeclampsia is reduced [32, 33]. The benefits of exercise prior to pregnancy for reducing the risk of preeclampsia, suggest that weight reduction interventions should feature well-designed physical activity components that focus on fitness, nutrition and weight loss in preference to weight loss alone.

This study is limited by its small sample size and low participation rate of eligible women (<50%) in the baseline survey and there is a possible downward and upward response bias to self-reported variables including current smoking status, alcohol use, fruit and vegetable intake and levels of physical activity. Despite the low participation rate, the cohort was well characterised at baseline with standardised body and biochemical measurements and the internal validity of the associations found between baseline risk and later hypertension in pregnancy should be high. A potential limitation of the study is the ability of perinatal data collections to report maternal health indicators accurately. A Canadian validation study found that gestational hypertensive disorders were more accurately coded (sensitivity 88%) than other variables, for example fetal/birth asphyxia (sensitivity 14.3%) [34]. The study is further limited by lack of data regarding whether or not women had a hypertensive disorder during a previous pregnancy as risk factors for recurrence are likely to be different for those women whose pregnancy is affected for the first time. The use of measured, rather than self-reported, anthropometric variables is an advantage in the study particularly because of the importance of body mass and central adiposity in predicting hypertension in pregnancy.

Our study findings have enhanced our knowledge base, in general, about risk for hypertension in pregnancy. These data demonstrate high levels of obesity and poor metabolic health among Aboriginal and Torres Strait Islander women of child-bearing age in north Queensland that increase the risk for hypertensive disorders of pregnancy. Levels of physical activity were low. The study was undertaken in remote communities situated in isolated areas of Cape York and on Torres Strait Islands that frequently have limited resources for access to secure supplies of healthy foods and safe exercise environments. As well as improving food supply, consideration should be given to improving community infrastructures to include identified facilities (gymnasiums, walking paths and improved night lighting) which provide physical activity programs aimed at improving the health of young women prior to their first pregnancy and between pregnancies. Beneficial effects of better metabolic health of a woman of childbearing age may also have a strong positive effect on the health of her children across their lifespan.