As long as you are around 8 cm or higher baby has enough room to grow and practice breathing... and since you are far enough along, if it gets under 8 cm doc will probably just conitue watchful waiting (with twice weekly NST and one of those being a full BPP) and if baby shows distress or you get to 5 cm or lower he may talk about baby time.

You really wouldn't worry about any major underlying medical problem. Sometimes low fluid just happens. And as long as you are being watched it should be ok. And rest a lot, drink a lot of fluid as ordered, you will be fine...