I've had an untreated prolactinoma for about 3 yrs and just found out I'm six weeks pregnant. In what ways, if any, will this affect my pregnancy?

Carefully monitor. This is definitely something to bring up with your OB doc, if you haven't already. They may want you to see an endocrinologist. There are many issues to consider. If the prolactinoma was small, you will probably be ok. But the pituitary will get bigger during pregnancy; beware of headaches, visual changes, vomiting, signs of a mass. If necessary, bromocriptine can be used during pregnancy.
See below. Once you are pregnant, there is nothing you need to do - as you do not want to use the medications for prolactinomas during pregnancy.