BACKGROUND:Familial patterns in reproductive outcome have been suggested previously, but few studies have comprehensively evaluated both length of gestation and types of growth retardation. METHODS:Information on intrauterine period and birth characteristics for a cohort of Swedish women born 1955-1972 was linked with information on these women's own reproductive experiences during 1973-1990. Familial trends in preterm deliveries, small-for-gestational age (SGA) births and two types of growth retardation were evaluated for mothers relative to their own birth characteristics (n = 4746), relative to their older sisters' deliveries (n = 2931) and among consecutive deliveries of the mothers (n = 14,209). Adjusted relative risks (RR) were calculated from logistic regression analyses. RESULTS:Mothers who had themselves been preterm at birth were not at increased risk of any of the outcomes studied. Mothers who had themselves been SGA at birth had an almost 50% higher risk (NS) of giving birth to either a preterm or an SGA infant than had mothers who had not been SGA at birth. Mothers tended to repeat the same patterns in subsequent deliveries: RR was 3.7 for a second preterm delivery given a previous one and 7.8 for a second SGA delivery given a previous one. Among SGA siblings, chronic growth retardation was more often repeated than was acute growth retardation. Mothers with an older sister who had given birth to a preterm infant had an 80% higher risk of giving birth to a preterm infant. CONCLUSIONS:Familial trends in gestational age and body proportions at birth were demonstrated; however, the relatively small population attributable risk per cents in Sweden are discussed.