The antenatal echographic diagnostic aspects and anatomopathological features of two foetuses suffering from sirenomelia, or caudal pole regression syndrome, are examined. Conduction of the two cases is compared. In the first case, observed at the 22nd week, antenatal diagnosis of sirenomelia led to the therapeutic interruption of pregnancy. The second case, seen close to term, closed with the birth of an already dead sirenomelic foetus.