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This study was based on 551,478 mothers with at least two singleton infants and 209,423 mothers with at least three singletons, first infant being born in 1967 or later. The mother-sibling units were analyzed as two family sets, so that for mothers with two or more infants the first two were analyzed, and for mothers with three or more infants the first three were analyzed. The family sets were thus not mutually exclusive, but the fallacy of ‘fixed sibship size’ was avoided (Golding J et al. Analysis of completed reproductive histories: a cautionary tale. J Epidemiol Community Health 1983;37:78-81)