Multiple congenital abnormalities (MCA) represent the most severe category of structural birth defects, (i.e. congenital abnormalities [CA]). Unfortunately, most MCA are not recognized and/or identified as MCA syndromes or MCA associations in the clinical practice. The term unclassified MCA (UMCA) is used for this category of MCA. We decided to evaluate the component CA of UMCA cases. The population-based large dataset of the Hungarian Case-Control Surveillance of Congenital Abnormalities (1980-1996) was evaluated. 'False' MCA, such as complex CA, polytopic field defects and sequences were excluded from the category of MCA. In addition, MCA syndromes caused by chromosomal aberrations and major mutant genes with preconceptional origin were excluded from the dataset of the Surveillance. MCA syndromes caused by teratogens and MCA associations with well-defined component CA were also excluded in the study. Thus, only UMCA cases (i) without the recognition of previously delineated MCA syndromes (ii) and/or without the identification of new MCA syndromes or (iii) caused by random combination of CA were included in the study. We compared data from 1349 cases with UMCA, 2405 matched population controls without any CA, and 21 494 malformed controls with isolated CA. There was a higher rate of stillbirth and a moderate male excess in UMCA cases, a somewhat shorter gestational age at delivery and an obvious reduction in birthweight. The intrauterine fetal growth retardation and rate of low-birthweight newborns showed an association with the number of component CA in UMCA cases. A similar association was not found with gestational age and the rate of preterm birth. UMCA represent one of the most severe categories of CA. The degree of intrauterine fetal growth retardation depends on number of component CA in UMCA cases.
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