so much bad advice here, you absolutely tell them. Put your self in the employers shoes not to be harsh but its a commercial world and you will be a very costly employee for them to take on and i doubt if you'll have a job waiting after the baby anyway i would tell them prove your worth so then you do have a job for when you can begin working again down teh track.
lieing from the start is going to hold you in very poor stead, not to mention if u dont tell them and decide to say something in say 1-2 months your still in trial period and can be terminated for no reason. (unless SA is different of course)