I really think that honesty is the best policy here. If you open about your pg now, he will likely believe you when you say that you didn't know when you signed your contract. Lets face it, they're going to find out anyway, you're probably better off looking like a responsible, honest employee from the word go. You can explain that you're still going to give 110%, up until the time of your maternity leave, and prove to them that you are great at what you do! I'm sure it's uncomfortable for you, but it will only become more so as time goes on. Also, you don't want anyone *guessing* and speculating before you have the chance to be upfront with your boss........

HTHs, good luck, let us know how you go.