Hi I Haven't Had A Regular Periods For The Last

Hi I haven't had a regular periods for the last few months. My OB wants me to start taking birth control but first to take norethindrone for 10 days. My question is what if I get my period before the 10 pills are done. Do I stop taking them ? Because he said the same day I start my period I have to start my birth control. So if I get my period before my pills the norethindrone is done do I stop taking it? Or would I take both while I'm on my period?.