Objective: Analysis of the results from registration of congenital defects in children of mothers taking therapeutic drugs in the fi rst trimester of pregnancy and comparison with a control group in the Czech Republic in the period of 1996-2001. Design: A retrospective analysis of data from the registry of congenital defects in the Czech Republic. Setting: Institute for the Care of Mother and Child, Prague. Methods: Analysis of incidences of selected types of congenital defects detected in newborns of mothers taking therapeutic drugs during the fi rst three months of pregnancy. The paper employed data from the all-state registration of congenital defects held in the Institute of Medical Information and Statistics of the Czech Republic in the period of 1996 – 2001. Data from healthy children born to mothers who took therapeutic drugs during the fi rst trimester of pregnancy were used as a control set. Results: In the period we observed, a total of 17,674 cases of children with diagnosed congenital defect were detected in the registration of congenital defects in the Czech Republic. In this number there were 784 cases of children whose mothers used therapeutic drugs during the fi rst trimester of pregnancy. The control group included 1,034 women who gave birth to healthy children without congential defects, although they took therapeutic drugs during the fi rst trimester of pregnancy. Statistically signifi cantly higher risk was found in 13 groups of diagnoses: anencephaly, inborn hydrocephalus, spina bifi da, inborn defects of eyelids, lacrimal system and orbita, anoftalmus, microphthalmus and macrophthalmus, inborn defects of ear, congenital defects of the heart septum, congenital defects of great veins, cleft palate with cleft lip, congenital defect of gall bladder, biliary pathways and liver, congenital deformities of the hip, reduction deformities of upper extremity, congenital defects of muscular and skeletal system. Signifi cantly higher risks were found for fi ve groups of therapeutic drugs: anticoagulants, antihypertensive drugs, peripheral vasodilatants, urological drugs and antiepileptic drugs. Conclusion: It is obviously impossible to draw signifi cant conclusions with clinical consequences on the basis of these results. Nevertheless, our results supplemented international databases of untoward effects of drugs and the conclusions may become a part of data set necessary for analysis of possible teratogenic effects of drugs used in the critical developmental period during the fi rst trimester. The pregnant women should avoid, during the fi rst trimester, all drugs except those, which are carefully medically indicated and accepted as adequately safe. The administration of other drugs should be evaluated by a clinical geneticist, who should take into account genetic and teratogenic risk in individual cases.

        Key words: congenital defect, population teratology, teratogen, incidence, Czech Republic