Hello Thanks for your query Fetal pyelectasis (FP) refers to a prominence / dilatation or the renal pelvis in utero. Fetal pyelectasis can be a relatively common finding in an antenatal ultrasound scan, often detected at the routine 2nd trimester morphology scan. It affects male fetuses more . The estimated prevalence is at ~ 2% of routine second trimeter scans Pyelectasis can result from a number of factors. In the majority of cases, it is physiological and resolves spontaneously, so you need not worry too much. However, it may also herald the presence or evolution of renal tract pathology, such as : fetal PUJ obstruction fetal VUJ obstruction urethral obstruction - e.g. posterior urethral valves vesicoureteric reflux duplex kidney Associations with trisomy 21: presence and strength of this association is currently under debate and at best considered very mild. Following are the cut off values for defining prominent fetal renal pelvis : 4 - 4.5 mm at 18 - 20 weeks (i.e. the routine 2nd trimester) 5 mm at ~ 20 - 29 weeks 6 mm at 32 weeks persistent fetal pyelectasis : 7 mm in the 3rd trimester The vast majority of cases (~ 96%) with mild pyelectasis in the second trimester resolve, either during pregnancy itself or in the early postnatal period, however serial monitoring with ultrasound is the best approach, you need to worry if there is increasing degree of pelvic dilatation in-utero progression bilateral involvement Please act in concert with your treating obstetrician and also seek the opinon of a pediatric surgeon, or a neonatologist, who would also guide you accordingly . Your case is borderline ( very mild ) as the cut off values suggest. Please dont worry.