Master of Midwifery - Research
Department of Nursing - Faculty of Health & Behavioural Sciences
Cooper, Emer, Analysis of growth data for breastfed infants and its relevance to breastfeeding, M.Mid.-Res thesis, Department of Nursing, University of Wollongong, 2003. http://ro.uow.edu.au/theses/200
Monitoring infants growth in the first year of life is a complex issue. It has been almost twenty years since the last Australian research was published on the topic of infant feeding methods and growth and no studies have compared Australian breastfed infants with the U.S. National Center for Health Statistics (NCHS) 1977 reference longer than three months. The NCHS Hamill et al. (1977) standard is the World Health Organisation (W.H.O.) recommended guideline currently accepted in Australia. It was decided to explore a sample of breastfeeding women of European descent and their infants from metropolitan Sydney and semi-rural NSW. The research hypothesis is, that the mean weight and length of breastfed infants, is significantly less at 1 year than the NCHS reference (Hamill et al. 1977) currently in use and with the revised Center for Disease Control (CDC) 2000 reference chart. Therefore the null hypothesis is that the mean weight and length of breastfed infants at 1 year are equal to the true mean weight and length of the Hamill et al. (1977) NCHS reference currently used at the time. The study is not representative of all breastfeeding mothers as subjects were attendees of Child Health Clinics who were following advice of Child and Family Health nurses. Methods: In order to test this hypothesis the weight and length of 300 breastfed infants was collated for each month between 1 month and 12 months of age and put into the format of a mathematical slope. Sections of the slope in a time series were compared e.g. 1 month to 3 months, 3 months to 6 months and 6 months to 12 months respectively. The reasons for this comparison in a time series are - 1. The literature review provided evidence that between 1 month and 3 months, breastfed infants grow faster than the current NCHS (1979) reference. 2. Between 3 months and 6 months breastfed infants growth rate begins to slow down when compared to the current NCHS (1979) reference. 3. Between 6 months and 12 months, breastfed infants continue to grow at a slower rate that the current NCHS (1979) reference. Following the collection of data, a series of slopes on a graph for weight and length was prepared for the comparison between male and female breastfed infants and the NCHS (Hamill et al.1977) data and the CDC (2000) revised data. The slopes of the graphs were compared mathematically using a statistical format and a p value of (less than)0.05 was considered significantly different. The gradient was measured using a 95% confidence interval statistic on the slopes of the graphs for weight and length between 1 and 3 months, between 3 and 6 months, and, 6 and 12 months and compared with the NCHS (1977) data and the revised version CDC (2000). Results: When the breastfed group was compared to the NCHS (1977) reference between 1 month and 3 months, male and female infants grew at a faster rate than the NCHS reference. The breastfed group continued to be above the average weight gain of the slope suggested by the revised CDC 2000 reference also during this time period. Between 3 and 6 months only the weight of both sexes is significantly different and it begins to slow down at this time, growing slower than both the NCHS and CDC average. The CDC 2000 guide also remains faster than the breastfed group between 3 and 6 months. Between 6 and 12 months, only the female breastfed infants weight gain was slower at the end of the first year and was significantly different to the NCHS reference. Male weight and length grew at a similar rate to the NCHS (1977) reference. Compared with the slope of the revised CDC 2000 reference between 6 - 12 months, both male and female breastfed infants gained weight at a slower rate, p (less than)0.05. Using the statistical t test on the terminal figures at 12 months demonstrated that there was not a significant difference at 1 year between the mean weight of the breastfed infants group, the NCHS and the revised CDC 2000 reference - p (less than) 0.05. It was assumed from testing the null hypothesis at 12 months, that Australian breastfed infants when measured against the NCHS (Hamill et al. 1977) and the revised CDC 2000 reference do not vary at the age of 1 year as much as previously thought. This may be due to the possibility that previous research did not record the breastfeeding management of the infants in their studies. Discussion: If infants were breastfed to a schedule of three or four hours they would not have the opportunity to gain weight at the same rate as infants who had free access to breastfeeding. The infants in this study were recorded as having been exclusively breastfed, fully breastfed or breastfed to need and may have had the opportunity to gain more weight. Fewer observations were available for length than for weight as time progressed which implies that most infants are weighed, but only some are measured. The implication of these results - from the data presented in this study- is, if clinicians use the existing charts as a guide to monitor adequate (good) weight gain of breastfed infants, that these infants will be seen as not gaining adequate weight or failure to thrive after the age of 3 months. This in turn has a potential to lead to the suggestion for offering breastmilk substitutes particularly after the age of three months, unless a thorough history is taken on the womans breastfeeding management. A serendipitous finding in this study was that Australian formula-fed infants may be heavier than breastfed infants. Two possible explanations for this were suggested - that parents are overfeeding, used heaped scoops of powder i.e. - intentional overfeeding -, or that parents are following the guidelines on the label of the formula tins and these volumes are inflated.