Change in social status and risk of low birth weight in Denmark: population based cohort study

To estimate the risk of having a low birthweight infant associated with changes in social, environmental, and genetic factors.

Population based, historical cohort study using the Danish medical birth registry and Statistic Denmark's fertility database.

All women who had a low birthweight infant (< 2500 g) (index birth) and a subsequent liveborn infant (outcome birth) in Denmark between 1980 and 1992 (exposed cohort, n = 11,069) and a random sample of the population who gave birth to an infant weighing > or = 2500 g and to a subsequent liveborn infant (unexposed cohort, n = 10,211).

Risk of having a low birthweight infant in the outcome birth as a function of changes in male partner, area of residence, type of job, and social status between the two births.

Women in the exposed cohort showed a high risk (18.5%) of having a subsequent low birthweight infant while women in the unexposed cohort had a risk of 2.8%. After adjustment for initial social status, a decline in social status increased the absolute risk of having a low birthweight infant by about 5% in both cohorts, though this was significant only in the unexposed cohort. Change of male partner did not modify the risk of low birth weight in either cohort.

Having had a low birthweight infant and a decline in social status are strong risk factors for having a low birthweight infant subsequently.