I am 22 weeks months carrying. during ultrasound scan. report show baby's kidneys have mild pyelectasia measuring 4to 5 mm. pls explain what it means?

Usually nothing: It means that there is a little urine trapped in the kidney. It has a very low association with downs syndrome. You should get a second trimester screening test but i wouldn't be worried. You should also get a followup scan in 3rd trimester to make sure the kidneys look okay but the vast majority of the time, they are. I would not lose any sleep of this! good luck with the rest of your pregnancy.