Patient: Hello Dr. Liu: I am 7 months pregnant now, and I had an ultrasound the day before yesterday, and there is a note on it that the fetal left kidney is 0.8cm wide. I would like to ask you whether this problem is serious and whether it will affect the growth and development of the baby in the future. I am looking forward to your reply! Thank you very much. Liu Yukun, Department of Obstetrics, Second Affiliated Hospital of Sun Yat-sen University: The cause of mild dilatation of the renal pelvis in normal fetuses may be due to compression of the ureter when the fetal bladder is overfilled, or it may be due to the influence of maternal progesterone, which slows down the smooth muscle peristalsis of the ureter, resulting in mild dilatation of the renal pelvis. Pathologic causes of pelvic dilatation include urinary tract obstruction and vesicoureteral reflux. It is generally accepted that an anterior-posterior dilatation of the renal pelvis greater than 15 mm is highly suggestive of obstructive pathology, while 10-14 mm is more likely to be pathologic, and most scholars recommend follow-up postnatal examination during the neonatal period. When the anteroposterior diameter of the renal pelvis is 4-10 mm, many cases are not pathologic and may be normal or physiologic.