thank you for using healthcare magic,
precum might have some sperm but since you are using IUD chances of getting pregnant is very less..
brown discharge can be because of some infection which occur commonly with IUD so consult gynecologist..
I will suggest you to wait for periods .. if it doent come till one week after expected date then you can go for urine pregnancy
you can take more confirmatory test like beta hcg
level or ultrasound
.. if pregnancy gets confirm consult gynecologist and you can go under abortion
just by pills as you will be in early stage and if you want to continue pregnancy you must remove IUD.. be under supervision of nearby gynecologist..