I mean that, besides its efficacy on mood control, lithium carbonate also has some side effects. Among them, lithium carbonate is thought to be toxic for the foetus. And it could also cause premature abortions if taken while pregnant.
Again, a pregnancy could trigger a post-partum depression that could worsen your present disease.
On the other hand, lithium carbonate won't give you any problems regarding fertility. So you could decide to get pregnant after discontinuing lithium therapy with no risk for you or for your baby.