Research confirms: Growing Up in New Zealand cohort broadly generalisable to all contemporary New Zealand births

Growing Up in New Zealand is the first longitudinal child cohort study to broadly generalisable to all contemporary New Zealand births, according to a scientific paper published in the Australian and New Zealand Journal of Public Health.

The study tracks the life course development of 6,853 children born in 2009 and 2010 - about 11% of all New Zealand births during this period. It aims to provide unique information about what family and environmental factors shape children’s early development in the context of contemporary New Zealand. This evidence can be utilised to target interventions at the earliest opportunity to give every child the best start in life.

The generalisability of the recruited cohort is important to ensure the findings from the Growing Up in New Zealand study provide population relevant evidence to policy makers who are creating new strategies and evaluating the reach of existing ones.

Comparing information collected from the Growing Up in New Zealand mothers with Ministry of Health data for all New Zealand births between 2007 and 2010, the paper found that birth characteristics of both groups, including gender, proportion of multiple births, mode of delivery and the age of mothers were generally closely aligned.

"This paper confirms how relevant the data we collect is for our policy makers, and all children growing up in New Zealand today. Growing Up in New Zealand is well placed to deliver findings that can inform policy development that is relevant to the diversity of the contemporary NZ child population," says study director Associate Professor Susan Morton.

As no register of all pregnant New Zealand women was available from which to recruit to the study, mothers were approached through multiple methods in a geographically defined area. Eligible pregnant women in the three Auckland, Counties Manukau and Waikato District Health Board regions were invited to enrol their as yet unborn infants using the help of health professionals, information in multiple public areas and widespread advertising in the media.

"The potential scientific cost of a geographical approach is that the generalisability to the population of interest, meaning all current New Zealand births might be limited," explains Dr Morton. "Therefore we made considerable efforts to track recruitment of mothers into the study to ensure that they best reflected the ethnic and socio-economic diversity of children born throughout New Zealand rather than just in the three recruitment DHB areas."

A record of the most recent NZ births available at the time of recruitment (years 2003-2007) was used as a reference population to measure the success of the recruitment strategies throughout the twelve month recruitment period.

"This paper demonstrates that these recruitment strategies were successful, and that - as planned - the information from the Growing Up in New Zealand cohort is able to address the issues that challenge the health and wellbeing of  our diverse New Zealand children."

The results in detail

The paper found that the proportions of the cohort that were a singleton versus a multiple birth and the gender balance were not statistically significantly different from the national birth data.

For both the Growing Up cohort and the national data the median maternal age at delivery was 31 years with 28% of mothers living in households in the most deprived quintile of areas.

The mode of delivery of the children was also closely aligned in both groups, showing small differences only in 2009 when slightly more New Zealand children were delivered by caesarean section or other assisted birth.

The research found that there were slightly less children with low birth weight or born preterm in the cohort compared to all New Zealand births at that time. This may reasonably be explained by the requirement that infants needed to survive to at least 6 weeks of age to become part of the cohort. Neonatal mortality is highest in low birth weight and premature infants.

The cohort was also more ethnically diverse than all national births. The mode and timing of data collection is likely to have contributed to these differences with the Growing Up in New Zealand information about expected ethnicity of children collected from parents before the child’s birth, and the national data collated several weeks after the children were born. As ethnic diversity in New Zealand is increasing, having a slightly more ethnically diverse group of children from recruitment will provide some protection to the cohort’s ability to continue to inform cross-sectoral policy in a population-relevant manner beyond the children’s early years.

View the paper online