Birth weights in three Norwegian cities, 1860-1984. Secular trends and influencing factors.

Data from birth records from three maternity hospitals in Norway have been used to study the trend in birth weight in this country from 1860-1984. The investigation is based on a sample of 200-300 records taken at random from 2 to 5 years around every 10th year, from each of the three maternity hospitals--amounting to a total sample of 9152 women. Besides describing the trend in birth weight, I have analysed the different factors influencing birth weight--using multivariate linear regression methods. The results show that the mean birth weight has changed remarkably little throughout this period of 120 years. The total increase has only been just below 200 g. From Montreal, Ward and Ward (1984) have reported a decrease in mean birth weight of about 430 g during the second half of the last century. The Norwegian data indicates a simultaneous fall in mean birth weight of about 70 g. This fall is, however, found mainly among unmarried women. Thus, the great decline in birth weight which is found in Montreal, is not apparent in our material from Norway, despite the fact that Oslo was industrialized at approximately the same time as Montreal. Of the various independent variables used in the multiple regression analyses, only the following variables appear to be of any importance: the year when the birth took place, the mother's menarcheal age, her marital status, the sex of the child and the parity number. In this material, I have found a linearly increasing birth weight from birth number 2 up to at least parity 8, and a larger increase from parity 1 to 2. The birth weight shows a significant increase in the case of married women as opposed to unmarried. This effect is most marked before 1900, indicating that the social conditions gradually became less unequal after the turn of the century. The birth weights have a decreasing tendency with higher ages of menarche. This tendency is constant throughout the whole period of time. Boys appear to be about 112 g heavier than girls, this difference being largely the same throughout the investigated period.