Methods for the evaluation of social effects on birth weight-- experiences with Swedish population registries.

All births in Sweden in 1986, registered in the Medical Birth Registry, were analyzed with linked data from the November 1985 census in order to identify variables which could be used as proxy variables for the socio-economic situation of the women, using birth weight as outcome variable. Two phenomena appear to be strong socio-economic indices of significance for birth weight: whether the woman co-habited or not at the census and whether she had a long education (> 14 years) and an "academic" work. The two variables interacted, however: when the woman had a long education, cohabitation status played no significant role. Maternal education as judged from occupation seemed to play a more important role than paternal education or job. To some extent, the effects of the socio-economic variables could be explained by different smoking habits but a residual effect existed which may have different explanations. Cohabitation status and education may be used as estimates of socio-economic level of significance for birth weight and perhaps other pregnancy outcome in the analysis of, for instance, the effect of various occupational exposures on pregnancy outcome.