What Is Use Of Giving Norethisterone After Miscarriage?

hello i`m 23,i had a miscarrage on the 20.04.11 i had retained product of conception the hospital tryed me a medcial managemant didnt work, so on the 04.05.11 i had a d&c still i had rpoc on the 31.05.11 i had a second d&c and still rpoc now they have put me a norethisterone for 3 weeks and a scan on the 05.06.11 i do now understand why i`m on norethisterone or why we have a wait 3 weeks to do another operation please could you help.thanks