摘要：Objectives: To describe the natural history of pyelectasis from its detection in the second trimester to delivery, its capability to predict renal pathology and whether prenatal development of pelvic dilatation is correlated to its postnatal evolution. Study design: A retrospective analysis involving 375 fetuses with a complete urological follow- up. Prenatal ultrasound was correlated with the results of postnatal investigation and frequency of postnatal surgery was established. Results: Two- hundred and eighty cases underwent at least two examinations before birth. 73.1% were male fetuses. 57.4% had a bilateral pyelectasis. Prenatal evolution of pelvic dilatation was the following:18.6% of the cases normalized, in 34.6% of the cases the dilatation reduced but did not disappear, in 30.7% it remained unchanged, while it worsened in 16.4% . One case from the first group, three cases from the second, seven cases from the third and 11 cases from the fourth needed surgical treatment. 1.9, 7.2, 18.6, 23.9% of cases respectively worsened after birth in the four groups (trend: P=0.001). Conclusions: Prenatal diagnosis of pyelectasis improves the outcome of these children due to a surgical approach that avoids renal damage. There is a good correlation between prenatal evolution and postnatal outcome, although a postnatal follow- up is opportune in those cases that normalized before birth.