How To Postpone Periods That Are Already Delayed?

I want to post pone my periods. i was expecting my periods on this month 20th and now it is been 3 days, still i have not got my periods. now i want to delay it one week more. As mentioned in the down link information, it was seen that taking regestrone tablets will help you to delay periods. but since i my expected date of periods is already done, will taking the mentioned tablet will help me to postpone periods? or is there any other tablets which work better?