Twenty-two year follow-up of New Zealand very preterm babies born in 1986

Professor Brian Darlow Cure Kids Professor of Paediatric Research Department of Paediatrics University of Otago Christchurch

Background to Research

Very low birthweight infants (birthweight <1500g – VLBW) account for only 1% of live births but between 50%-75% of the work load of neonatal intensive care units (NICUs). Survival for this group is now >90% but what is most important for individuals, their families and society, is the quality of that survival in the longer term.

In 1986 we enrolled all New Zealand VLBW infants, who were cared for in NICUs, in a prospective study. The cohort was last followed up at 7-8 years of age when 5% had severe disability, 5% had moderate disability and 15 % had mild disability.

Recent overseas studies of former VLBW infants in young adulthood have assessed health and mental status, functional outcome, and quality of life. There are concerns about health issues including hypertension, renal impairment and respiratory problems. There are few population based studies and no New Zealand data. In this project we want to establish the feasibility of re-contacting the cohort and to administer a questionnaire and obtain consent for a more comprehensive assessment.

What are your research objectives?

The ultimate objective is to assess health, welfare and educational achievements of the 1986 national cohort of infants born with a birthweight <1500g at 22 years of age, compared with controls. The cohort was last assessed at age 7-8 years and the first requirement is to trace surviving individuals. The objective is essentially a feasibility study to re-establish the cohort, obtain consent and administer a questionnaire. This will enable us to apply for the more extensive funds needed to fully assess the cohort by psychometric testing, visual examination and medical tests.

How will you conduct your research?

Step one will be to retrace the cohort of former very low birthweight infants born in 1986, of whom 338 survived and were discharged from hospital. By 7-8 years 12 further children had died, 17 were traced to an overseas address, seven declined to take part in further follow-up and four were untraced. We were able to follow up 298 children (96% of survivors resident in New Zealand, 91% of all survivors). Children were traced from their original addresses, through their grandparents’ addresses, via their GPs, from the electoral role and in a limited number of cases by newspaper advertisements. In 1994 the NHI system was in place and we now have NHI numbers for the full cohort as well as their parents’ and grandparents’ addresses at that time. A check will be made with Statistics New Zealand to find out if any individuals have died since the last follow-up. We expect most of the cohort to be traced via the latest address we have on file, but a few will undoubtedly require much more time consuming efforts.

Step two will be to establish an appropriate control group of individuals born at term in 1986. To ensure that the control group is recruited on a national basis we will identify controls by sampling from hospital birth records for 1986. We plan to use the birth records for the five level III centres existing in 1986 (i.e. National Women’s, Waikato, Wellington, Christchurch and Dunedin). Together these five hospitals accounted for 72% of all in-born infants in the original VLBW cohort, and can be considered broadly representative of the population of births in 1986. Controls will be selected by sampling at random from the hospital birth register.

It is anticipated that controls will be harder to trace than the original VLBW cohort and may also be more resistant to participation in the study. Under these circumstances it is likely that proportionately more controls will have to be sampled to achieve a similar number enrolled compared to the VLBW cohort. Nevertheless we are hopeful that with careful attention to detail it may be possible to attain a 70% recruitment rate. Any differences in baseline characteristics between the VLBW and control cohorts (e.g. gender, ethnicity, family socio-economic background) will be allowed for statistically.

Step three will provide a questionnaire to the cohort and controls to assess educational attainments, occupation, quality of life measures, self-assessment of health status, major health problems/disability and social functioning. We will also consider approaching the parents for consent (with the participant’s permission) and administer a questionnaire to them.   

 Are there others in your field in NZ or globally conducting similar research?

There are now an increasing number of overseas studies reporting on outcomes of former very low birthweight infants in young adulthood. However most of these studies are for children born in the 1970s or early 1980s, few are population based and there are no New Zealand data. The 1986 NZ cohort was one of the first national studies of very low birthweight infants from any country.

If you achieve your objectives what will that mean to those suffering from the disease or to the knowledge advancement of this disease?

 These data will provide a platform for the development of further research applications to conduct a more comprehensive assessment of long-term functional outcomes for VLBW infants. Overall the study will give unique information on outcome from VLBW in New Zealand and will facilitate improved interventions to lessen many problems resulting from preterm birth. Overseas experience suggests that allowing former VLBW young adults provide self-assessment on their achievements, not only gives a very different and more positive picture from the assessment of others, but is also empowering for the individuals concerned.

Is there national or international collaboration on your research project?                                     

The study is a collaboration between an academic neonatologist (Brian Darlow) and a biomedical statistician who is a Senior Research Fellow with the longitudinal Christchurch Health and Development Study (John Horwood) from the University of Otago Christchurch, and an academic psychologist whose research has covered both the impact of premature birth and psychological functioning in adolescence (Lianne Woodward) from the University of Canterbury. We are in contact with other groups interested in similar studies overseas but have no direct collaboration with them.