Polyhydramnios usually means that there is an increased amount of amniotic fluid present. The amount of amniotic fluid is usually determined by fetal urination and fetal swallowing, so anything that impairs either of these can impact the amniotic fluid.
The amniotic fluid increases until about 34 weeks and then slightly decreases toward the due date.
The most reliable method of determining the amnount of amniotic fluid is the Amniotic Fluid Index and many doctors will do this test first in order to make the right diagnosis. At 32 weeks, for example, the normal AFI is between 86 mm and 242 mm.
When polyhydramnios is found, your doctor usually does some additional tests to see if a cause can be found.