Education, infant mortality, and low birth weight in Sweden 1973-1990: Emergence of the low birth weight paradox

Aims: Educational differences in infant mortality, birth weight, and birth weight-specific infant mortality in Sweden were analysed. The "low birth weight paradox", where low birth weight infants have a lower mortality risk if born to women of lower rather than higher social strata, was addressed. Methods: The study includes about a million single births 1973-90 to women born 1946-60. There were 6,544 infant deaths and 35,334 low birth weight infants. Analysis conducted on six-year time periods was restricted to 652,859 births to women aged 25-32 at the time of delivery. Odds ratios and 95% CI were estimated by logistic regression. Birth weight-specific infant mortality rates were calculated by education. Results: Infants of women with low/low intermediate education had significantly higher odds ratios than those of highly educated women of being of low birth weight or of dying. If one compares only the infants of women with low and high education, these differences were accentuated over time. The low birth weight paradox appears over time. Conclusion: The widening differences in infant mortality and low birth weight over time may be caused by the decrease in women with low education, signifying increased selection and growing social disadvantage in this group. The emergence of the low birth weight paradox suggests that the distribution of causes of low birth weight differs between educational groups, and further that these causes are differently related to infant mortality. To disentangle these two groups of causes and their effects on infant mortality seems highly relevant.