Ah thank you for the images! I think ureterocele would lead to oligohydramnios since the fetus is still ingesting the amniotic fluid, but as Paris mentioned, it does not pee it back out due to the bladder jet being obstructed by the ureterocele! So the fluid is going into the fetus from the amniotic sac but is not returning to the sac after going through the urinary system.