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T-tests were used to compare mean values for infants of same birth order in the different (independent) family-groups. Standard analyses of variance were used to adjust the differences in mean birth weight for gestational age, and to evaluate and adjust for confounding by: Maternal age (<20, 20-29, 30-34, 35+), maternal education (<11 years, 11-14 years, 14+ years), marital status (single or cohabiting at first and second birth, others), maternal diabetes (yes/no), interpregnancy interval (6 categories) and time period (1967-76, 1977-86, 1987-98). In these models the independent variables were treated as categorical fixed factors, and each birth order was analyzed separately.