First off, I'm NOT a doctor and have not had a kid but I have seen a few people paddle until they were quite far along with no obvious issues other than the PFD starting to get a bit snug! I think it's mostly about listening to your body. If later in the season it's getting to be a challenge with mobility/energy, there is always the calling option, that can't be any more jarring than running while pregnant which a lot of women do.
Usually from what I've seen with my friends and other sports the standard advice is they can keep doing what they were doing before although a couple were told to watch that their heart rates didn't get too high (these were runners).
There is a brief discussion here...
http://answers.yahoo.com/question/index?qid=20080429112635AAWcwx2 I'm sure there is more stuff out there.
I'd say uber-competitive stuff may be a bit much but you never know, everyone is different! There are definitely lots of benefits to remaining active while pregnant as long as you listen to your body and be sensible, that's provided there aren't already any underlying complications or risks associated with the pregnancy which the doctor should be able to outline.