Research suggests Maori women are more likely to get breast cancer than European, Asian or Pacific women despite a tendency to give birth earlier and have more children - factors that normally reduce breast cancer rates.
A study, released today by the University of Otago in Wellington, found the difference between breast cancer rates in Maori and European or other women had increased from 7 percent in 1981-1986 to 24 percent in 2001-2004.
The rate of breast cancer in Maori women increased from 123 to 210 per 10,000 women, while for European women it rose from 114 to 170.
"It is still a bit of a mystery why Maori rates have increased 70 percent compared to 50 percent for European/Other over the research period, and at this stage we can only speculate on some of the possible reasons," lead researcher Ruth Cunningham said.
"Given what we know, one would expect Maori to have lower rates than European women as they give birth earlier, have more children and lower rates of hormone therapy use. All these factors normally reduce breast cancer rates."
Dr Cunningham said post-menopausal obesity could be one explanation.
"It's likely that there are environmental factors not yet determined which are contributing to the differences in rates," she said.
"It's also possible that there could be biological factors which may help to explain the high rates among Maori women, but very little work has been done in this area."
The study showed Asian and Pacific women had the lowest incidence of breast cancer in New Zealand, with rates rising from 70 to 126 per 10,000 and 112 to 141 respectively.
The Asian breast cancer rate, while the lowest in the study, was high when compared with that of most Asian countries, suggesting environmental factors were having an impact, Dr Cunningham said.
Low Pacific rates were consistent with reduced risk factors such as having more children at a younger age and lower alcohol consumption.
Dr Cunningham said more research was needed on the role of risk factors and the biological nature of breast tumours.
The study also found that women from high socioeconomic groups were more likely to get breast cancer than those from low socioeconomic groups because they had less children, were less likely to breast feed and more likely to use hormone therapy.
Breast cancer kills about 600 women in New Zealand a year.