Intergenerational birth weight associations by mother's birth order - the mechanisms behind the paradox: a...

Mother's birth order is inversely associated with offspring birth weight despite positively associated with the mother's own birth weight. The causes behind this relation have not been elucidated.

To investigate the relation between mother's birth order and birth weight of her offspring, with emphasis on possible mechanisms behind the findings.

Population based cohort study over two generations.

Data were from the Medical Birth Registry of Norway, based on all births in Norway, 1967-2006 (2.3 million births). Units where both mothers and offspring were singletons and offspring were first born were included, forming 272,674 mother-offspring units for the analyses.

Birth weight in the second generation.

Mother's birth weight increased steadily with increasing birth order from 3369 g for first born to 3538 g for fourth or later born mothers. In contrast, there was a monotonic decrease in offspring mean birth weight with increasing mother's birth order (9.1 g per birth order (95% C.I.; 6.8, 11.4)). First born mothers tended to be older, to have higher education, to more often be married or cohabiting, and to smoke less than later born mothers at the time of their first pregnancy.

The general reduction in mean birth weight among first born mothers was not observed in the next generation. We suggest that first born mothers have the same biologically potential for achieving similar sized offspring as later born mothers, and that social factors account for the inverse relation.