Aim of study: An analysis of quarterly occurrence of selected types of birth defects in the Czech republic in the 1994 – 2005 twelve-year period. Evaluation of possible changes of occurrence of selected types of birth defects along with standard deviations from mean values calculated from 1974 – 1993 period data. Type of study: Retrospective analysis of quarterly data from the National Birth Defect Register in 1994 – 2005 period. Material and methods: Mean incidences of birth defects from the previous 1974–2003 period (per 10 000 live births) were calculated and compared to current data from each particular quarter. A calculation of observed/expected rates (O/E ratio) for particular selected birth defects was performed. Further, results are presented by means of figures and are widely discussed. Results: In 1994–2005 period, totally 1 132 567 children were born in the Czech Republic. Out of this number, 38 298 children were born with one or more birth defects. The mean incidence of birth defects was 338.15 per 10 000 live birth in the period under the study. Our results make clear, that incidences of particular defects as well as incidences as a whole are influenced by several factors making possible incidences increase. These factors are extremely difficult to assess individually and separately, therafore a particular period and a particular population is to be considered. Conclusions: Incidences of 11 selected types of birth defects and their time-changes are analyzed. Current registration of birth defects and their incidences analysis as a vital part of a environmental factors assessment is underlined.

        Key words: birth defect, anencephaly, spina bifida, congenital hydrocefalus, cleft palate, cleft lip/palate, omfalocele, oesophageal defects, anorectal malformation