Perinatal mortality is closely related to infant birth weight. However, the examination of both gestational age and birth weight provides a more comprehensive approach. This study presents a method of evaluation which is readily applicable. Data were obtained from the computerized files of the maternity ward of the University Hospital of Caen. This report is based on case-reports of 17,952 single births, from May 1980 to July 1985 (cases of multiple birth have not been considered since their age/weight relationships are usually different). All stillborn infants and those who died within the first week of life were considered. Any infant whose gestational age was 28 weeks or more but who showed no sign of life at birth was considered stillborn. Gestational age was determined from the mother's last menstrual period, providing it concurred with the obstetrical evaluation (i.e. echographic measurements of foetus before 20 weeks of gestation). If not, the clinical evaluation was retained. Results are presented in the form of blocks, each one representing 1 week of gestational age and a birth weight of 500 g. The perinatal mortality rate was calculated for each block. Blocks of similar mortality rates (i.e. with no significant difference) were then pooled by means of a recently published method [3]. This difference between the mortality rates of 2 blocks (in percentages) may be measured by means of the chi-square test. The test enables the evaluation of the "distance" between two adjacent zones, which can be calculated for all the blocks of the graph, by pairs.(ABSTRACT TRUNCATED AT 250 WORDS)