I have taken 2 tests already one yesterday and it was negative. However when I was pregnant with my son I took 4 and they were all negative and it turned out I was almost 5 weeks pregnant. Is there and explanation for this? When should a test be positive if my last period started on sept, 7 2010 and my cycles normally 28 days.

Most women will get a positive result if they are pregnant once they miss a period. For some women, however, it just takes a bit longer for there to be enough hcg hormone in their urine to be picked up.

You could be pregnant, you could just have an off cycle. Give it another week and try another test if still no period. (be sure to follow directions exactly) If negative, either wait until next month and see what happens with your period or see your doctor about having a blood test done.