How To Prevent My Wife From Pregnancy After Knowing That

How to prevent my wife from pregnancy after knowing that on 27 and 28 February, 2016 she didn't have woman's period? Or is this factor causes my wife to have a child or other factors, because she used to tell me that she didn't sleep well all night, I think that this is the factor that my wife didn't have menstrual period on 27 and 28 February 2016. Can you explain me about this factor?
I am looking forward to seeing your answer, Dr.