Excuse to Avoid White Water Rafting

I am 6 weeks pregnant and will be about 10 weeks pregnant when my hubby and I go on vacation. We signed up early to go white water rafting with some friends, but obviously I don't plan to go now that I'm pregnant. However, because it's early in the pregnancy, we don't want to tell them I am pregnant yet. We need an excuse to get out of it. Can anyone help us come up with one??