One reason to keep an eye on low amniotic fluid is to see if bub is processing the fluid through his/her kidneys. Especially as you have poly-cystic kidneys yourself, I would imagine they will be keeping a close eye on the development of bub's kidneys too. Low amniotic fluid CAN be (but doesn't always mean) that the kidneys aren't developing properly. I would ask your OB if he's checking the kidneys out during the scan.