I Got Married Sep 27.. I Have Always Regular Period

I got married sep 27.. i have always regular period oly... Dec 8 my period date but last month date will change 13 jan.. now still i not get period, i check the pregnent test but result negative.. then i check doc she give duphaston tablet for five days... i have chance to get pregnent? I do sex with my husband now? I get fear wen i get pregnent pl advice clearly... wen do sex for getting pregnent..