Within four years, 53 miscarriages due to chromosomal abnormalities after IVF and ICSIwere diagnosed and analyzed. Preliminary investigations revealed chromosomal translocationsin two male partners, and chromosomal aberration was assumed in one further malewho survived chronic lymphatic leukaemia. Nearly 80% of the miscarriages were causedby trisomies for various autosomes. Chromosomes 16 and 22 were observed most frequently.Polyploidies were detected in nearly 20% of the cases. Each patient who had amiscarriage in a fresh or frozen cycle was paired with a control patient who became pregnantduring the same period of treatment. Inclusion criterion for the control group was alive birth in a fresh or frozen cycle following embryo transfer. Critical risk factors for miscarriage(p<0.05) were maternal age at the time point of oocyte retrieval und total FSHdosage and, in the t-test, also the number of metaphase II (MII) oocytes. The value for antimullerian hormone (AMH) was not correlated with the risk for miscarriage. In the twocouples with known translocations in the male partners, cytogenetic chorionic villi analysisrevealed trisomies of autologes other than those of chromosomes affected by fusion. Embryoquality was classified in A, B, C and D according to the degree of fragmentation. Neitherembryo quality nor the duration of culture for 2-3 or 5-6 days nor the number of transferredembryos was a risk factor for miscarriage. The results are not surprising becauseaneuploidy in oocytes increases with maternal age. Optimizing the intrafollicular environmentfor oocyte maturation may improve meiotic spindle function and thus reduce agedependentaneuploidy in oocytes. It can be speculated that this is the mechanism and reasonwhy more ?competent? embryos develop and pregnancy and live birth rate improve bymetformin treatment in patients at risk for gestational diabetes, e.g., PCOS patients.