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Based on the hypothesis that maternal-fetal genetic differences in membrane transport and signal transduction may influence intrauterine development, the recent acquisition on transport function of Rh protein prompted us to study the relationship between joint maternal-fetal Rh phenotype and birth weight. Considering that metabolic effect of maternal-fetal competition could be amplified by environmental conditions, we have investigated possible seasonal effects on such relationship. We have studied 5291 infants born in Sardinia in the period January 1993--December 1996 and 984 infants born in Rome during 1996. In Rh(-) mothers there is a significant association between season of birth and birth weight that shows the highest mean value in infants born in autumn (i.e. conceived in winter). The association is much more evident in male than in female infants. In male infants from Rh(-) mothers, the association between birth weight and season is significant in Rh(+) male newborns only. Recent observations by our group in NIDDM suggest that glucose transport in RBC may be related to D protein, thus we propose an interpretation of the present observation in terms of transport function. When the density of D protein in the infant is greater than in the mother, the balance is in favour of the infant who may attain a significant developmental advantage when conceived in the cold season.