I was told this too at 35 weeks. I thought it was rubbish and insisted on another scan at 39 weeks. Was told there was 'no way' it could go back up but it bloody had! Still, though, they didn't want me to go to the midwife led unit and pushed me towards continual foetal monitoring and induction at 41 weeks. I ummed and aaahed and was eventually scared into agreeing after a consultant told me there was a small risk of it causing cerebral palsy.
In the end, my waters broke at home so didn't need an induction. I did have the monitoring and it wasn't that bad in the end. When my son was born he virtually drowned the midwife. So much for low fluid.
My feeling is that MoragG is right, it seems to be a very inexact science, so tell them to stuff it if you feel up to it, but if you're going to be worried about it, it might be easier to go along with it all. I was adamant about using the midwife unit (I was gutted not to be able to have a home birth so midwife unit was a compromise already) but I told the midwives how I felt and they were so lovely in making sure it all felt as relaxed as possible. I still had a lovely birth with low lights, music on, my own photos around the room, not being asked questions etc. It was just how I'd hoped it would be, and having the monitor on didn't mean I couldn't move about at all, which is what I had been scared of.
If you feel strongly about not having too many interventions etc, ask if you can have the pessaries before they just stick you on a syntocinon drip, and if you do have the drip, ask to start it off low. To be honest, I don't think I'd have coped without an epidural if they'd gone straight in with a high level synto drip as it really strengthens contractions.
Just remember it's not one thing or the other. There are plenty of ways you can still have the sort of birth you want without having to go out on a limb and worry about it, and as everyone tells you, you really don't give a toss about any of it once you've got your baby in your arms.
Hope it all goes well for you!