Looking at the NAMI article, it seems the risks are minimal with lithium during pregnancy in general (especially beyond the first trimester). How do you feel about the diagnosis? Did the symptoms begin during pregnancy or have they been ongoing here or there (or constantly) for a while?
I would ask your OB/GYN if she knows of any concerns before making your final decision, though the ultimate decision depends on you and your comfort levels. You'll want to consider not only the effects and risks during the last six weeks of pregnancy but also when you are breast feeding the baby (unless you've already made the decision to bottle feed). You'll want to consider how the impact will be on your child if your emotional/mental state gets bad enough. At the very least, I would strongly suggest beginning therapy regardless if you begin taking lithium or not.
I would avoid any medication until after the pregnancy and breast feeding. We are always finding new side effects of medications.