The requisites for filling up birth certificates in France (> or = 180 days of pregnancy and vitality at the registration of birth) are different from the criteria recommended by the World Health Organisation (WHO) (at least 500 g) for national statistics. The number of stillborn babies and neonatal deaths known from two different sources in the Nord-Pas de Calais Region in 1988-1989 have been compared ie: the vital statistics published by INSEE and the data of a survey carried out in hospitals according to the WHO criteria. Among the 808 known perinatal deaths in the survey, 305 (37.7%) were ignored or were not classified in the same categories by the registration services for vital statistics. 230 children including 29 born alive had not been registered and 75 children born alive had been registered as still born. The disparity in these definitions between INSEE and the WHO has serious consequences: the perinatal mortality measured by INSEE is underestimated by 28%, in comparison with mortality registered according to the WHO criteria. The 180 day threshold of viability reduces the social rights of employed women and makes it more difficult for them to have the money they spent on medical care refunded. Finally, as it is impossible to find a trace of these children who were not legally registered since they were under the threshold of viability, the mourning response of parents to perinatal deaths is all the more difficult. Current evolution in the threshold of viability observed in maternity units, makes it necessary to change the criteria which are applied for the legal registration of births.