The incidence of chromosomal abnormalities in cleavage-stage embryos produced in vitro, 50-70 percent depending on maternal age, is considerably higher than that of spontaneous abortions, indicating that a sizeable percentage of chromosomally abnormal embryos are eliminated before any prenatal diagnosis. In most in vitro fertilization (IVF) laboratories one of the powerful tools to improve results is embryo selection, based on morphological and developmental characteristics. FISH is currently the best method to analyze polar bodies and blastomeres since the former have poor quality metaphases and in the latter metaphases are produced in low rates even after culture. Mosaicism rates vary in the literature. Some of the differences between centers are attributed to the population; others to hormonal stimulation and the general quality of embryos produced in those centers. Recurrent miscarriage (RM) in patients with normal karyotype is defined as three or more consecutive spontaneous abortions of less than 20-28 weeks gestation.