Obesity is a heterogeneous group of disorders that have in common disturbances of energy balance and an increase of fat cells. Genetic and development factors, however, have an important role in making obese or not obese people. Recently there has been a surge of interest on the possible relationship of obesity with birth weight and with season of birth. We have studied the effect of season of conception and ACP1 phenotype (an enzyme associated with obesity disorders) on birth weight in a sample of 809 newborn infants from the Caucasian population of Central Italy. Birth weight of newborn conceived in the period January-April is higher compared to that of subjects conceived in the subsequent period of year. The effects of season of conception on birth weight is enhanced in newborns carrying the heterozygous ACP1BA phenotype. BA infants are also more represented among those conceived in the first months of the year. The observations suggest that environmental and genetic factors favouring reproduction in the early months of the year also favour the birth of well nourished offspring. These infants could have an increased susceptibility to over-weight during adult life in Western populations.