My wife is getting her periods on time from last one year. This month she didn't get her periods.?

Even after 20 days of her expected period date she still didn't get periods. We visited a gynaecologist a week back. Pregnancy test was negative but she told that she can not confirm this at this stage and gave Duphaston tablets for 5 days. She told if it is pregnancy it will protect it and if not she would get her periods. Even now she didn't get her periods. Can somebody please tell us what could be the reason? Any thing to worry or is it pregnancy? I checked for pregnancy symptoms in few website but I notice only 50% of them in my wife. Also, I read that Duphaston is banned in many countries. Is it true? Does using Duphaston have any negative effects?