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We aimed to discuss the prenatal diagnosis and pathological features of sirenomelia, and to review current embryogenic theories. We observed two sirenomelic fetuses that were at the 19th and 16th gestational week respectively. In the former, transvaginal ultrasound revealed severe oligohydramnios and internal abortion, whereas bilateral renal agenesis, absence of a normally tapered lumbosacral spine, and a single, dysmorphic lower limb were detected in the latter. In both cases, X-rays and autoptic examination allowed categorization on the basis of the skeletal deformity. Subtotal sacrococcygeal agenesis was present in both cases. Agenesis of the urinary apparatus and external genitalia and anorectal atresia were also found. Classification of sirenomelia separately from caudal regression syndrome is still debated. Recent advances in the understanding of axial mesoderm patterning during early embryonic development suggest that sirenomelia represents the most severe end of the caudal regression spectrum. Third-trimester ultrasonographic diagnosis is usually impaired by severe oligohydramnios related to bilateral renal agenesis, whereas during the early second trimester the amount of amniotic fluid may be sufficient to allow diagnosis. Early antenatal sonographic diagnosis is important in view of the dismal prognosis, and allows for earlier, less traumatic termination of pregnancy.