ObjectiveTo evaluate the importance and evolution of isolated, mild fetal pyelectasis, detected in early pregnancy by high-resolution transvaginal sonography, and to determine its association with abnormal fetal karyotypes.MethodsTransvaginal scan at 11–16 weeks' gestation and transabdominal ultrasound examinations at the time of amniocentesis (16–18 weeks) were performed in 1093 pregnant women undergoing genetic amniocentesis because of advanced maternal age. In 795 cases, transabdominal scans were repeated at 22–24 weeks. Women were excluded if they had a spontaneous abortion, chose to terminate their pregnancy, or declined amniocentesis. Each patient was screened for fetal pyelectasis, defined as an increase in anteroposterior renal pelvic diameter, using cutoff values related to various stages of pregnancy.ResultsIsolated fetal pyelectasis was detected at the first ultrasound examination in 56 women (5.1%) in early pregnancy, in 32 (2.9%) at the time of amniocentesis, and in 23 (2.9%) at 22–24 weeks' gestation. Two fetuses with diagnoses of mild pyelectasis at the first transvaginal ultrasound demonstrated abnormal karyotypes at amniocentesis. In one case, the pyelectasis disappeared at 22–24 weeks' gestation.ConclusionThis retrospective study shows that pyelectasis is more frequently detectable by high-resolution transvaginal sonography in the first half of pregnancy than in the second half. When detected in early pregnancy, the finding is frequently transient and not associated with an increased risk of abnormal fetal karyotypes.