Why Is Norethisterone Prescribed For 20 Days After A Miscarriage, And Is It Less Effective If I Stop After 10 Days If Bleeding Has Stopped?

Hi. Since november when I had misscarage i didn t stop bleeding. Didn t bleed much but brown discarge. Doctor perscribed me Norethisterone for 20 days 3x a day 1 tablet. Why for such a long time? Can tretment be less efective if I stop eatting them after 10 days. I stoped bleeding on day 9 and on day 11 I saw slightly brown discarge again at the evening. Thank you.