I would say that yes it is common, but we don't usually see it. There is water in there filling up all the space in the uterus- and that water has to come out sometime. Usually the water just seeps out and the mum licks it away as quickly as it comes.
However, in my case, I have a momma cat that actually had her water break this weekend- a bunch of odorless, colourless fluid gushed out and then more slowly seeped out for several hours afterwards.
