"It is a generally accepted assumption that sporadic pregnancy losses occurring before an embryo has developed represent a ‘physiological’ phenomenon, which prevents conceptions affected by serious structural malformations or chromosomal aberrations incompatible with life from progressing to viability. This concept is supported by clinical studies in which embryoscopy was used to assess fetal morphology prior to removal by uterine evacuation. Fetal malformations were observed in 85% of cases presenting with early clinical miscarriage. The same study also demonstrated that 75% of the fetuses had an abnormal karyotype. Fetal chromosomal aneuploidies arising from non-inherited and non-disjunctional events are common. Indeed, in a recent study using comparative genomic hybridization to study the chromosomal complement of all blastomeres in preimplantation human embryos, more than 90% were found to have at least one chromosomal abnormality in one or more cells. The clinical implications of minor, mosaic and possibly ‘transient’ aneuploidies remain unclear. However, while most fetuses with severe developmental defects will die in utero some aneuploidies can be compatible with survival to term. The most commonly encountered is trisomy 21, although 80% of affected embryos perish in utero or in the neonatal period. In most cases, the extra chromosome is of maternal origin and caused by a malsegregation event in the first meiotic division. The risk of this increases with maternal age and may be considered to be a biological rather than pathological phenomenon. Although fetal chromosomal aberrations may be identified in 29% to 60% of cases in women with RM, the incidence decreases as the number of miscarriages increases suggesting other mechanisms as a cause of the miscarriage in RM couples with multiple losses."