I slept on plastic (under the sheet with a towel inbetween) for the last month or so as I was so paranoid about this. I also put a towel on the car seat as I'd also heard about it ruining car seats.

In the end I didn't need either. My waters broke after several hours of labour at home and several more at hossy. It happened when the midwife was checking how dilated I was. I had been checked on arrival and was 6cms and it didn't break but it happened later on. The poor midwife got such a shock. I had to get changed as I was absolutely drenched in it.