The relation between the parity of the mother and the birth-weight as well as the size of the new-born child, taking into account the sex, were examined as follows: From the patients needing obstretical care during the period of 1966 to 1971 those 3441 women were chosen who had born twice or more times during this time. By means of an electronic data processing system the weight and size differences of the children were correlated with the parity of the mother taking again into account the order of male and female births. Births with uncertain gestational age, non-cephalic presentations, and premature births were excluded (WHO definition or gestational age less than or equal to 266 days counted from last menstruation). 1. The average birth-weight increases by 105 g from the first to the second child, by 39 g from the second to the third, and by 12 g from the third to the fourth child. Obviously, there is no linear link between the increase of the birth-weight and the increasing parity. The major weight-increase is to be observed from the first to the second child. 2. A corresponding statement concerning the size of the new-born children cannot be made which is partly due to the relative inaccuracy of the method applied. 3. On the average boys weigh between 130 g to 150 g more than girls according to the parity.