In this study the substantial and in part contradictory data available in the literature was collected concerning the frequency of small supernumerary marker chromosomes (sSMC) in the human population in general, and in special subpopulations. One hundred and thirty-two studies on sSMC were reviewed. In summary 1,288,693 cytogenetically studied cases detecting 980 sSMC were compiled. In 132 international surveys there were no ethnic effects detected in the sSMC frequency. sSMC were present in 0.075% of unselected prenatal cases but only in 0.044% of consecutively studied postnatal ones. In infertile subjects, 0.125% were sSMC carriers, distinguishing male from female subjects by a 7.5:1 difference in sSMC frequency for this special group. In developmentally retarded patients the sSMC rate was elevated to 0.288%, similar to prenatal cases with ultrasound abnormalities (0.204%). No increased risk for the presence of sSMC was detected in ICSI-induced pregnancies. Worldwide there are ≈2.7x106 living sSMC carriers; 1.8x106 have a de novo sSMC and ≈70% of those are clinically normal. Strikingly, 30-50% of pregnancies diagnosed with an sSMC fetus are terminated. This may be connected with the empirical risk that ≈30% of sSMC carriers manifest clinical abnormalities. Thus, in summary there is a strong need for a better genotype-phenotype correlation enabling better genetic counseling.

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