To provide data about the phenotypic appearance of the embryo of early failed pregnancies after IVF. Clinical prospective descriptive study. Tertiary care center. Twentythree women who had conceived by IVF and had a missed abortion before 12 weeks of gestation. Embryoscopic examination of the embryo before curettage. Cytogenetic analysis of the chorionic villi by standard Gbanding cytogenetic techniques or by comparative genomic hybridization in combination with flow cytometry analysis. Embryonic phenotype and karyotype were determined. Twentyone of 23 IVF embryos showed structural defects on embryoscopic examination. Seventeen of 23 specimens had a chromosomal abnormality. The majority were numerical aberrations such as monosomy X (2 cases). Trisomies for chromosomes 18 (one case), 16 (three cases), 15 (one case), 14 (two cases), 13 (one case), 12 (one case), 11 (one case), 10 (one case), 9 (one case), 8 (one case), and 3 (one case)were observed. A structural chromosome anomaly leading to a chromosomal trisomy was observed in one case. Aneuploidy explained the grossly abnormal embryonic development documented by embryoscopy in 15 of 21 cases. Aneuploidy is the major factor affecting normal embryonic development in missed abortions after IVF. Further investigation is needed to elucidate mechanisms that might prevent normal embryogenesis but evade detection by the cytogenetic techniques used in the present study. To provide data about the phenotypic appearance of the embryo of early failed pregnancies after IVF. Clinical prospective descriptive study. Tertiary care center. Twentythree women who had conceived by IVF and had a missed abortion before 12 weeks of gestation. Embryoscopic examination of the embryo before curettage. Cytogenetic analysis of the chorionic villi by standard G-banding cytogenetic techniques or by comparative genomic hybridization in combination with flow cytometry analysis. Embryonic phenotype and karyotype were determined. Twenty -one of 23 IVF embryos showed structural defects on embryoscopic examination. Seventeen of 23 specimens had a chromosomal abnormality. The majority were numerical aberrations such as monosomy X (2 cases). Trisomies for chromosomes 18 (one case), 16 (three cases), 15 (one case), 14 (two cases) 13 one case, 12 one case, 11 one case, 10 one case, 9 one case, 8 one case, and 3 one case were observed. A structural chromosome an Aneuploidy explained the grossly abnormal embryonic development documented by embryoscopy in 15 of 21 cases. Aneuploidy is the major factor affecting normal embryonic development in missed abortions after IVF. Further investigation is needed to elucidate mechanisms that might prevent normal embryogenesis but evade detection by the cytogenetic techniques used in the present study.