Environments For Healthy Living (EFHL) Griffith birth cohort study: characteristics of sample and profile of antenatal exposures

Health is the product of a complex interaction of factors relating to societal norms, a person’s local physical and social environment, and their biological and psychological capacities [1‐3]. While public health researchers and practitioners recognise the importance of a multi-level, ecological model of disease causation and health promotion, [4, 5] there has been limited research successfully quantifying these relationships in way that can support population based interventions [6]. The primary reason for this is that most previous aetiological and implementation research designs are not consistent with the multi-level logic that underpins our understanding of disease causation.

In order to advance the ecological understanding of disease causation, epidemiological research needs to be able to quantify the contextual factors as well as the proximal risk factors responsible for relevant health outcomes. There is a growing interest throughout the world in developing study designs that enable this level of multi-level analyses [7].

Environments for Healthy Living (EFHL) is a birth cohort study being conducted in South East Queensland, Australia, designed specifically to operationalise the ecological model of disease causation [5] and thus provide information required to develop policy driven improvements in population health. Using methods previously described, [8] data collected in this study include biological samples, participant surveys, medical record and administrative data linkage, land use and spatial variables. With a planned total cohort of nearly 4000 mother/infant dyads being recruited from a circumscribed study region, comprising communities across a broad range of socioeconomic exposures, this study has the capacity to address questions of contemporary importance.

The aim of this paper is to provide a comprehensive baseline profile for the first five recruitment years of the EFHL birth cohort and determine the ability to analyse the repeated cohort samples separately or combined. The study sample characteristics and maternal, household and, community exposures are described. Variation and data stability over recruitment years is examined.

The results of the analyses of the baseline questionnaire and birth details provide a robust basis for developing a valid and useful epidemiological quantification of the ecological determinants of health. The large study sample is broadly representative of the majority of deliveries in the region, with sufficient range of exposure data to support on-going analyses of research questions relating to the social determinants of health. Temporal and cohort effect analyses indicate strong data stability across five recruitment years.

The three public maternity hospitals from which the study recruited, accounted for 78% of all births in the study population during 2006–2010. The remaining 22% of births in the study region occurred either at private hospitals, birthing centres, or were home births and were not represented in the EFHL study sample. The proportion of public hospital deliveries in the study population was higher than the national average of 70% [27]. Public hospitals are known to serve a disproportionately larger share of patients of relatively low socio-economic status [30]. Women giving birth in public hospitals have been found to be younger, a higher proportion are first births, a greater proportion smoke, and more women present with medical conditions such as hypertension and diabetes [31]. The Health Districts of the study region are known to have higher proportions of socio-economic disadvantage and more people with non-English speaking backgrounds than the national average [9]. These characteristics were similarly reflected in the EFHL sample who demonstrated lower incomes shares, younger age, more overseas born and high proportions of smoking than the national average.

The findings related to the birth sample characteristics suggested that while sampling was broadly representative of the target study population, there were some differences. The EFHL sample included births with greater gestational age, more singletons, higher birth weights, and fewer still births than that of the study population. This study sampled women who were of 24 weeks gestation or later, in waiting rooms attending their routine antenatal clinic visits at public hospitals, during the latter half of the calendar year (except in the 2006 pilot year). Not all antenatal clinics were able to be attended by the research midwifes and short patient waiting times in some clinics prevented the approach of all women, which is likely to have contributed to only half of the estimated births during the recruitment periods being approached for the study. In addition, routine antenatal clinic visits typically include women assessed as low risk births carrying singletons or twins, excluding higher multiple pregnancies or those women with known health conditions associated with an increased risk of birthing complications (mainly premature babies <28 weeks gestation). Two studies of South East Queensland births data have shown some seasonal differences in pre-term births, birth weight, and limb length in full-term singleton babies, with heavier babies born in the winter months which coincide with our recruitment periods [32, 33].

The repeated sampling design was used in this study to enable the assessment and quantification of the impact of structural or environment changes and health policy implementation during the total recruitment period. The results of the analyses presented here in relation to changing prevalence of alcohol consumption during pregnancy suggest this repeated sampling design is a key strength of the study. Foetal alcohol syndrome and the consumption of alcohol during pregnancy have been the increasing focus of research nationally and internationally, [34‐36] resulting in changes to national practice guidelines in 2009, [37] and the development of various state media campaigns in recent years targeting young women, alcohol, and pregnancy [38‐40]. Australian studies have found approximately 50%-60% of women drink some alcohol during pregnancy [34, 35, 41]. However, there is a need for temporal examination of prevalence patterns of consumption to have insight into the effectiveness of policy dissemination [42]. The findings of this EFHL study present the first repeated prevalence measure of alcohol consumption during pregnancy in the same geographic population over a five year period. The results demonstrated a 9.9% reduction in the consumption of alcohol during pregnancy from 2006 to 2010. While beyond the scope of this paper, further analysis will be conducted with regards to changes in risk groups, periods of consumption, frequency, quantity and binge drinking.

The stability of the repeated sample recruitment process can be assessed by comparing the baseline characteristics and antenatal exposure data for each annual cohort. Few of the socio-demographic baseline characteristics of the repeated samples showed significant differences over the five years of data examined. Pre-pregnancy BMI increased over the five calendar years reflecting trend changes found in the national population for the same time period [43]. Caution must be taken with a few individual exposures that demonstrated change over time and a single factor that showed instability in the pilot year when data collection methods were being established (number of children living in the household). However, these maternal and socio-demographic factors are not seasonal, and are therefore unlikely to be affected by differences in the months of recruitment. The overall results of this analysis indicate strong robustness of the data and a high degree of stability.

This paper presents a comprehensive baseline profile of the first five recruitment years of the EFHL birth cohort. Stability of data ascertainment across the recruitment years is strong. The broad representation of socio-economic status, community measures and key proximal exposures such as tobacco, alcohol, and drug intake during pregnancy provides the range of exposure required to ensure identification of effect for relevant risk factors as study outcomes become prevalent. This study, with multiple cohorts from repeated sampling, has the potential to assess health policy implementation during the study period and make important contributions to population health.