Abstract

Introduction

Previous reports from northwestern Ontario (2) and British Columbia (3) have shown increased high birthweight rates among aboriginal newborns compared to those in the general population, but it is not clear whether this is a general phenomenon or if these rates are increasing.Furthermore, the implications of such findings are also not known. The purpose of this study was to compare high birthweight rates between northern (predominantly aboriginal) and southern (predominantly non-aboriginal) populations in Saskatchewan and to determine if these rates have changed over time. We hoped that this information might provide additional insight into the effects of acculturation on carbohydrate intolerance in aboriginal peoples.

Methods

For the purpose of this study, rates were compared for northern and southern Saskatchewan. Northern Saskatchewan was defined as the region of the province that is under the administrative jurisdiction of Northern Health Services Branch of Saskatchewan Health (roughly the northern half of Saskatchewan). Approximately 66% of the 30,000 people living in this region for the years under study were aboriginal, whereas at least 85% of people living in the south were non-aboriginal.(4) Thus, we believe that our comparison of high birth-weight rates was effectively between those of aboriginal and non-aboriginal descent (although an ecologic study does not prove individual characteristics). Furthermore, any observed differences in high birthweight rates between northern and southern Saskatchewan were minimized rather than accentuated because of the population distribution noted above (to the degree that such differences are attributable to aboriginal versus non-aboriginal descent).

In addition to comparing overall high birthweight rates between the two regions for 1975 - 1988, we used the data for those years to compare high birthweight rates on the basis of maternal age, parity, paternal age, sex of child, gestational age of child, number of previous stillbirths of mother and type of birth (singleton or more than one). Differences were analyzed using the chi-squared statistic. To compare trends over time between the two regions, three-year moving averages were used rather than annual rates because of yearly fluctuations in the smaller population of northern Saskatchewan. We did not use tests of significance to compare temporal changes (trends) within each of the two total populations.

Results

Table 1 shows high birthweight rates for the other factors listed above. Male sex of child, increasing gestational age, increasing parity of mother, singleton births and no history of previous stillbirths were all associated with more high birthweight infants in both the northern and southern parts of Saskatchewan. There was a tendency for this association to exist with increasing maternal and paternal age in both geographic areas as well. However, in all categories and subcategories, high birthweight rates were greater in the north. Furthermore, rates increased and increased more rapidly during the time of the study for infants of northern mothers who were older and who had had more previous children (Table 2).

Discussion

Why do these differences exist? Macrosomia (birthweight of 4000 g or more) is most commonly associated with maternal obesity, gestational age of more than 42 weeks, gestational diabetes (1) and male sex.(5) The latter does not explain our findings since there is little difference in male-to-female ratios between northern and southern Saskatchewan (6) and because northern female newborns also had increased high birthweight rates (Table 1). Differences in rates of prolonged gestation are also unlikely to explain our results because we observed an equal disparity in high birthweight rates between the two geographic areas when the gestational age was 38 - 41 weeks (Table 1). Thus, differences in rates of maternal obesity and/or gestational diabetes probably account for our observations. Independent genetic factors may also be possible.

Since both maternal obesity and gestational diabetes are risk factors for macrosomia and since maternal obesity is also a risk factor for gestational diabetes, it is difficult to discern the relative importance of obesity versus gestational diabetes in causing high birthweights. However, we suspect that pregnancy-induced maternal carbohydrate intolerance may be an important factor.

Tables 1 and 2 show that the highest rates of macrosomia in the north, combined with the largest disparity between north and south, occurred among older mothers and older fathers (the latter is presumably because older mothers have older mates). Increased maternal age is a risk factor for gestational diabetes. (1) Furthermore, our own findings from three communities in northern Saskatchewan (7) show that up to 14% of women have a history of gestational diabetes and that there is a high degree of correlation between community rates of gestational diabetes and female obesity. Finally, results from a recent national survey of infant feeding practices among Canadian Inuit and Indian women (8) showed that 6.1% of pregnancies were complicated by diabetes during pregnancy (range from 2% to 16%). This is in contrast to a 3% incidence of gestational diabetes in pregnancies among the general population. (9)

Why are high birthweight rates not only greater but also increasing more rapidly among aboriginal people? Although they may have genetic differences that lead to a propensity for larger newborns, there must be environmental factors that lead to the temporal changes that we are reporting. We believe that altered lifestyles due to acculturation are leading to escalating rates of obesity and that this is contributing to an unusually high incidence of gestational diabetes even before established diabetes mellitus significantly manifests itself within this population. (7) We suspect that high birthweight rates and those for gestational diabetes may be even more elevated among aboriginal people in southern Saskatchewan, where there has been longer exposure to the dominant culture.

There are other important implications from our findings. If they are due to an increasing incidence of gestational diabetes associated with acculturation, this may be contributing to the accelerating rates of non-insulin-dependent diabetes mellitus (NIDDM) among aboriginal peoples. In addition to causing increased perinatal complications, gestational diabetes is a predictor for NIDDM in at least 40% of women who have had it, (10) and it appears to be a significant risk factor for the development of NIDDM in their children. (11) If the main cause of our findings is maternal obesity, that provides further support for our hypothesis that increasing rates of obesity among aboriginal people in northern Saskatchewan are due to their progressive exposure to non-traditional lifestyles. (7)

In contrast to the recommendations of a recent Canadian task force, we believe that routine screening for gestational diabetes should be carried out in all pregnant aboriginal women so that optimal therapy can be instituted to prevent macrosomia and to identify women who might benefit from lifestyle interventions designed to prevent NIDDM. Strategies to prevent gestational diabetes in this population should also be developed since this may substantially reduce the risk for the development of NIDDM in successive generations.

References

2. Munroe M, Shah CP, Badgley R, Bain HW. Birthweight, length, head circumference and bilirubin level in Indian newborns in the Sioux Lookout Zone, northwestern Ontario. Can Med Assoc J 1984;131:453-6.

3. Thomson M. Heavy birthweight in native Indians of British Columbia. Can J Public Health 1990;81:443-6.

4. Saskatchewan Health. Saskatchewan Hospital Services Plan covered population 1982.
5. Statistics Canada. Percentiles of birthweight by duration of pregnancy-Canada (except Newfoundland). Ottawa, 1980.

6. Tan L, Irvine J, Habbick B, Wong A. Vital statistics in northern Saskatchewan, 1974-1988. Saskatoon: Northern Medical Services, Department of Family Medicine, University of Saskatchewan; 1992.

7. Dyck RF, Tan L, Hoeppner VH. Body mass index, gestational diabetes and diabetes mellitus in three northern Saskatchewan aboriginal communities [short report]. Chronic Dis Can 1995;16(1):24-6.

8. Health and Welfare Canada. National database on breastfeeding among Indian and Inuit women. Survey of infant feeding practices from birth to six months-Canada, 1988. Ottawa: Medical Services Branch, 1990.

9. Amankwash KS, Prentice RL, Fleury FJ. The incidence of gestational diabetes. Obstet Gynecol 1977;49:497-8.

10. Henry OA, Beischer, NA. Long-term implications of gestational diabetes for the mother. Baillieres Clin Obstet-Gynaecol 1991 Jun;5(2):461-83.

11. Pettit DJ, Aleck KA, Baird HR, Carraher MJ, Bennett PH, Knowler WC. Congenital susceptibility to NIDDM. Role of intrauterine environment. Diabetes 1988;37(5):622-88.

Author Reference

Leonard Tan, Department of Community Health and Epidemiology, College of Medicine, University of Saskatchewan