Objective: To point out possible discrepancies in the classic hypothesis of M€ ullerian development and to present cases that support an alternative hypothesis. Design: Case reports. Setting: University hospital. Patient(s): Six women with uterine malformation. Intervention(s): Surgical, clinical, and outpatient evaluation of the anomalies. Main Outcome Measure(s): Assessment of the genital anomalies according to our embryologic investigations and embryologic-clinical classification and standard classifications for M€ ullerian malformations. Result(s): We have included examples of cases presenting with didelphys uterus with normal cervix, didelphys uterus with septate cervix, and bicornuate (or didelphys) uterus with septate cervix and vagina versus septate uterus with double cervix and vagina, septate bicervical uterus with isthmic communication, and normal simplex uterus with septate cervix and vagina. Conclusion(s): These case reports prove that the fusion and reabsorption processes in the development of the M€ ullerian ducts can be affected to different degrees in the superior-convergent and inferior-divergent portions, thus resulting in atypical or transitional cases without a classification. Therefore, for a more appropriate classification of the uterine malformations, even if the same degrees of fusion, resorption, and development defects are used, a separate classification should be made for each superior and inferior uterine segment. (Fertil Steril Ò 2009;91:2369-75. Ó2009 by American Society for Reproductive Medicine.