What are the runs? Can you portage or simply get out if it feels too much?
I don't kayak, but I rowed Westwater at 12,000 one spring while 6 months pregnant. Without giving all the gory details, I dumptrucked myself and had a long, scary swim. Actually 2. I was lucky and I don't regret it, it's simply a part of my life. I think the in utero adrenaline rush made my kid (now 10) a fantastic athlete!
I will say that you should consider how tired you may quickly get exerting yourself while pregnant; it creeps up on you.
And, throw out "What to Expect When You're Expecting" and look for some books by Aviva Romm
Her wisdom is empowering.