Soar, if you are truly Bipolar you cannot wean off of Lithium without sooner or later beginning to cycle into manias and depressions. Just like a Type I Diabetic can't wean themselves off of insulin and magically have their pancreas start working.
Childbearing and the situation with meds CANNOT be handled here - you really really really need to talk to your doctor.
There are risks to you to go without a mood stabilizer if you have Bipolar Disorder. There are risks to a fetus (especially during the first 3 months) if you stay on a mood stabilizer.
Your really need to talk about this with doctor who handles your Bipolar Meds and with an Obstetrician. A genetic counselor wouldn't be a bad idea either!