Why has fetal renal pelvis dilatation in my one expectant twin left kidney only occurred at 32weeks gestation?

Im 32 weeks pregnant, expecting twin boys. I have scans every fortnight. At my 20 week scan no abnormalities were detected. At my 30 week scan again no abnormalities. Yet at my 32 week scan has shown that the one twin left kidney has renal pelvis dilatation measuring 10mm AP with evidence of calyceal dilatation. The liquid volume around the baby is ok and the bladder is still functioning. What has caused this and why so late during my pregnancy??