Using the Swedish Medical Birth Registry for 1983 the birthweights were studied from the 38th to the 42nd week of gestation. Several criteria were used in order to retain a "normal" population of infants without signs of pathology. The mothers all negated smoking. Of about 93 000 livebirths roughly 10% fulfilled the criteria. Both in the infants who were the first-born child (n = 5.640) and in those who were the second-born child (n = 3.599) there was a successively higher mean birthweight for each completed week until 41 weeks. After that a graph connecting the mean values showed a significant flattening.