What Causes Brownish Discharge During Menstruation?

Hi, i'm 5 week of pregnant.yesterday Night i saw Little Bit blood when i wipe my vagina.today i have Brown colour discharge .i went to dr.he took ultrasound and told everything ok.but end of next week can saw the heartbeat of Embryo .he gave me the untrogest tablet and told to take 3 Times 2 tablet a day.i am afraid to take.becouse in 2012 November i had miscarriage in 7 week of pregnant.that time also i had Brown discharge and after the started to bleed.dr told it was Missed abortion .but he did not give me any Mediation like this time.i am afraid of miscarriage .why i have Brown discharge and this untrogest can protect my pregnant? How to take this tablet after the meal Or before the meal.hope You will give me best answer