All over the web it is recommended that lithium be withdrawn, at least for the first trimester. Does your OB want you to be lithium free for a period of time before you actually attempt a pregnancy? There are a plethora of good drugs currently available. If your pdoc will provide you a list of possible meds, then you can take it to your OB and have a discussion regarding your options. It's always a risk to change an effective therapy, but I agree with your OB that lithium should be discontinued.

Best of luck!