Birth weight distribution as an indicator of environmental effects on fetal development.

A medical birth registry was used for a geographical analysis of birth weight distribution. Nearly 900,000 Swedish singleton births, 1973-1981, were used for an analysis of the effect of some variables and for standardization for these variables. A marked change in the rate of low birth weight infants (LBW, less than 2,500 g) was seen in the country between 1976 and 1977. A U-formed effect of maternal age and of parity was demonstrated. A marked interaction between the effects of these two variables existed. Two social groups were compared and the well-known high rate of LBW infants associated with low socioeconomic conditions was demonstrated. Standardization for the variables mentioned influenced this effect only little but reduced the difference between the social groups concerning infants above 3.5 kg weight. The background data were used for analysis of restricted geographical areas.