I just took an at home pregnancy test, and there is a very very faint line. i have had the mirena iud since july 2016. last week i was having some cra?

In theory, even a: "faint" line needs to be considered a positive. I recommend a repeat test when there's uncertainty or a low likelihood of pregnancy. Do the test again first thing in the morning and restrict fluids the evening before. If it's still even faintly positive, please follow up with a doctor. Complications can occur if you get pregnant with an IUD in place. Best wishes. Dr. Anne