Study population
The Aboriginal and Torres Strait Islander women in this study were participants in the March 1998-December 2000 Well Person’s Health Check (WPHC) cross-sectional health survey [19], which included 26 rural and remote Indigenous communities in the Bowen, Cairns, Cape York, Torres Strait and Mount Isa Health service districts. All adults were invited to participate through printed media, local radio and word of mouth via the local health service, community council and community groups. The survey was attended by 2862 Indigenous people aged 15 years and over, giving an overall participation rate of 44.5% (according to local census data) [20]; 51.7% were female. Participants overall, were not different demographically from the age and sex distribution of the Indigenous population who did not attend, based on census data. A subset of women from the original survey data were selected for this study because the WPHC occurred when they were aged between 15–44 years (a generally accepted range of childbearing age).
Data sources
Three datasets were linked to bring together information on maternal pre-pregnancy factors (anthropometric, biochemical and lifestyle) gathered at the WPHC, and the presence or absence of a hypertensive disorder during the next pregnancy following this health check. First, records of Aboriginal and Torres Strait Islander women of childbearing age who attended the WPHC (details of the survey are outlined below) were linked to Queensland hospital Unit Registration numbers by probabilistic matching of name, date of birth and residential address. The second dataset contained hospitalisations data (including International Classification of Disease (ICD) 9 and ICD 10 codes and adjacent diagnosis-related groups and descriptions) identified those women who were hospitalised for pregnancy-related conditions after the date of their WPHC up to the study censor date on 30 March 2008. To idenify women who had a hypertensive disorder of pregnancy after the WPHC, a third dataset provided by Statistical Output, Health Statistics Centre, Queensland Health was added. The third dataset included perinatal data specific to those women who had been hospitalised for birth of an infant of at least 20 weeks completed gestation or a birthweight >400 g when gestation was unknown [21]. When a woman was hospitalised for more than one birth during the follow-up period, only perinatal data for the birth nearest the health check was requested. Women who were likely to have been pregnant at the time of their health check were excluded based on the date of their health check, the date of their delivery and the estimated gestation of the pregnancy at the time of birth.
ICD codes within the perinatal data identified women who had hypertensive conditions (ICD O10-O16) during their pregnancy (Figure 1). The outcome measure “hypertension in pregnancy” in this study applies to women who were recorded in the perinatal dataset as receiving a diagnosis of one of the forms of the disorder; pre-existing hypertension (ICD O10), pre-existing hypertension with superimposed preeclampsia-eclampsia (ICD O11), gestational hypertension without proteinuria (ICD 013) and preeclampsia-eclampsia (ICD O14 and O15).
Well person’s health check
A standardised proforma was used for data collection in the baseline WPHC survey. Body measurements and specimen collection were performed by a multidisciplinary team of trained staff from the Cairns Tropical Public Health Unit (Queensland Health) and local health staff in each communtiy. Face-to-face interviews followed a structured questionnaire. Fasting venous blood samples were collected by a medical officer, registered nurse or trained phlebotomist [19].
Anthropometric measures
Participants were invited to attend in the early morning following at least an 8 h fast. They were weighed in light clothing to the nearest 0.1 kg on digital electronic scales and height was measured to the nearest centimetre. Body mass index (BMI) was calculated as weight (kg) divided by height squared (m2). Waist circumference was measured to the nearest centimetre. Anthropometric cut-offs were set according to World Health Organisation criteria [22].
Blood pressure
Seated blood pressure was recorded three times over approximately 10 min with an automated blood pressure monitor, and mean systolic and diastolic measurements were calculated.
Self-reported behavioural factors
Fruit and vegetable intake was self-reported by 24-h recall and 7-day recall was used to assess the duration and intensity of physical activity. The participants were categorised as having adequate physical activity if they reported a minimum of 30 min of moderate to vigorous exercise on at least 5 days in the week. Current smoking status was self-reported. Participants who drank alcohol self-reported the types and amounts of alcohol consumed in the week before the health check; they were then categorised as being safe drinkers, non-drinkers or harmful (“risky”) drinkers according to Australian National Health and Medical Research Council guidelines (2001). Risky drinking for women was defined as more than two standard drinks per day or more than four on any single occasion. The physical activity, smoking and alcohol intake measures have been widely used [23] and the 24-h recall dietary questionnaire and red cell folate measure have been validated against other measures of micronutrient intake and the quality of the diet with respect to fruit and vegetables [24, 25].
Biochemical measures
Early-morning fasting venous blood samples were collected in ethylenediamine tetra-acetic acid vacuum tubes and clotted serum separator vacuum tubes (spun for 10 min at 1000 RCF in a swinging bucket centrifuge and separated within 1 h). Samples were stored in 4-8°C and transported for analysis within 24 hours. The biochemical measurements included glucose, triglycerides, total cholesterol, high density lipoprotein cholesterol, γ-glutamyl transferase and red cell folate. Glucose and lipids were measured using photometric enzyme endpoint assay with Cobas Integra 700/400 (Roche Diagnostic, Switzerland). γ -Glutamyl transferase was measured by a kinetic photometric procedure with Cobas Integra 800 (Roche Diagnostic, Switzerland). An elevated level was defined as ≥ 50 U/L and < 50 U/L as normal, according to criteria established by Queensland Health Pathology Service (http://www.health.qld.gov.au/qhcss/qhps/default.asp). Red cell folate was measured with the Bayer Advia Centaur automated immunoassay system (Bayer, Australia) by the Queensland Health Pathology Service in Brisbane. The reference range for this assay was 295–1800 nmol/L.
Urine specimens collected in sterile 50-ml containers were either the first morning or at least 2 h after the most recent void. If protein was detected by dipstick urinalysis (Combur-test, Roche) or if the participant was known to have diabetes, hypertension or obesity (BMI >30 kg/m2), the albumin: creatinine ratio was measured by immunoassay in grams per mole. Microalbuminuria was defined as an albumin:creatinine ratio of 3.4-34 g/mol, and macroalbuminuria as >34 g/mol [26]. From May 1999 to the end of the project, the albumin:creatinine ratio was measured in all urine specimens.
Metabolic syndrome
The metabolic syndrome in this study is defined according to the International Diabetes Federation criteria as comprising waist circumference ≥ 80 cm plus two or more of the following: raised triglycerides (≥ 1.7 mmol/L), reduced high-density lipoprotein (< 1.29 mmol/L), raised blood pressure (systolic ≥ 130 mm Hg or diastolic ≥ 85 mm Hg) and raised plasma glucose (≥ 5.6 mmol/L) (http://www.idf.org/metabolic-syndrome).
Ethics
Ethics approval for the WPHC was provided by the Cairns Base Hospital Ethics Committee in March 1998. The participants provided consent to have their Queensland Health medical records reviewed and linked to their WPHC records. The current data linkage study was approved by the University of South Australia and Cairns Base Hospital Human Research Ethics Committees (June 2009 and September 2009) with support from the peak health councils representing Indigenous people in the region: Apunipima and the Torres Strait Islander and Northern Peninsula Area Health Councils.
Statistical analysis
The data were analysed in a generalised linear model (Poisson distribution) with robust variance estimates to calculate prevalence ratios (PRs) and 95% confidence intervals (CIs) with 2 sided p-values of 0.05 for baseline characteristics associated with hypertension during the first pregnancy after their health check. Data are presented in both unadjusted and adjusted analyses. An a priori decision was taken to adjust the model for age because hypertension is commonly associated with increasing age and, because it was expected that Aboriginal and Torres Strait Islander women would differ in their anthropometic and metabolic characteristics, ethnicity was included in the adjusted model. Further adjustment for months from participation in the baseline survey to time of admission for birth was not associated with the outcome and was not retained. Fruit and vegetable intake was excluded from the analyses due to very small cell sizes. The analysis was conducted with STATA 11 (STATAcorp, College Station, Texas, USA).