Sirenomelia, which has been reported for more than 450 years, is the most severe form of caudal dysplasia. This disorder is classically associated with agenesis or severe dysplasia of the urinary tract. A 27-year-old gravida 3 para 0020 presented at 19 weeks' gestation with monoamniotic twins, one normal and one with sirenomelia. The pregnancy miscarried following invasive diagnostic testing. Autopsy confirmed the findings of sirenomelia, meningomyelocele, omphalocele, persistent cloaca, and two normal kidneys. The abnormalities accompanying sirenomelia appear as a spectrum. The pathogenic theories of sirenomelia are reviewed in light of this case with normal kidneys.