Objective: To evaluate the impact of suggested change of stillbirth definition of newborns 500–999 grams on the total perinatal mortality in the Czech Republic. Design: Retrospective epidemiological analysis of stillborn fetuses of extremely low birth weight registered as miscarriages. Setting: Institute for the Mother and Child Care, Praha, Czech Republic. Methods: Comparison of trial nationwide registration of stillborn babies of 500–999 grams through the Czech Society of Perinatal Medicine in 2003 and 2004 with the official obligatory registration of miscarriages of the same birth weight at the Czech Office of Health Statistics and Information. Results: Comparing the database of aggregated national data on stillborn babies 500–999 grams and the database of individual data on miscarriages of the same birth weight we found that the number of stillborn babies in 2003 was 15% smaller than number of miscarriages (151 versus 178). This relatively small difference due to insufficient trial registration in one of 14 regions suggests quite good nationwide readiness to new still births registration after the definition is changed. Contribution of congenital malformations was the same in both databases (41%) it should not be included in new perinatal mortality, because these cases are rather results of successful prenatal diagnosis followed by termination of pregnancy than „adverse outcome“ of perinatal care. Even if the stillborn babies of 500–999 grams without congenital malformations are included, which would increase current perinatal mortality by 1.0%, the Czech Republic would still belong to countries with the lowest perinatal mortality in the world while following the WHO recommended definitions. Conclusion: Accepting the WHO definition which includes the stillborn babies of 500–999 grams into total perinatal mortality, which are until now registered as miscarriages in the Czech Republic, the total perinatal mortality would increase by 1.6‰. If the babies after induced termination of pregnancy due to genetic indications (diagnosed until 22nd week of gestation) are not included, the increase would be of 1.0‰only.

        Key words: definitions, stillbirth, miscarriages of 500–999 grams, congenital malformations, perinatal mortality