3 days late for period

I am 3 days late for my period my daughter well be 8 months the 18 of this month. And Since I got my period back when she was 4 and a 1/2 months then have been on time or one day late. I took a test the first day that I missed and negative. I didn't really test with my daughter until 14 days late. So how long should I wait before testing again? My period was supposed to start on Friday but I'm thinking I should wait until this Friday to test.