Recent trends in Canadian infant mortality rates: effect of changes in registration of live newborns weighing less than 500 g

Abstract

Objective: To ascertain whether the increase in the crude infant mortality rate in Canada in 1993 was due to a recent increase in the registration of newborns weighing less than 500 g as live births.

Design: Ecological study, with Poisson regression analysis.

Setting: Canada.

Subjects: All live births and infant deaths in Canada between 1987 and 1993, as reported by Statistics Canada. Data from Newfoundland were excluded because they were incomplete for 4 years.

Outcome measures: Proportion of live births by low-birth-weight category; and annual crude and adjusted infant mortality rates.

Results: Over the study period the proportion of newborns weighing less than 500 g registered as live births increased significantly (chi 2 for trend = 71.26, p < 0.01). This trend was an isolated phenomenon rather than a general increase in all low-birth-weight categories (chi 2 for trend in the proportion of newborns weighing 500 to 2400 g registered as live births = 1.14, p = 0.28). The crude infant mortality rate per 1000 live births decreased from 6.4 in 1991 to 6.1 in 1992 and then increased to 6.3 in 1993. Poisson regression analysis revealed that the apparent increase in the infant mortality rate was caused by the increased registration of infants weighing less than 500 g as live births. The adjusted infant mortality estimate for 1993 was lower than that for 1992.

Conclusions: The increased infant mortality rate in Canada in 1993 appears to be due to increased registration of infants weighing less than 500 g as live births. Comparisons of infant mortality rates by place and time should be adjusted for the proportion of such live births, especially if the comparisons involve recent years.