saifra,
When I was your wife's age, I remember that even really regular periods (mine were every 23-25 days like clockwork) could occasionally become irregular. I did occasionally miss a period, but I wasn't pregnant.
My understanding is that ovulation takes place 2 weeks
before the next period, not two weeks
after the last one. Thus, allowing for occasional irregular periods, it isn't always possible to know from a pregnancy test whether you are pregnant if you are assuming that this month was supposed to happen on time. In other words, wait a week or two, and if her period hasn't started, take another pregnancy test. I hope that you have a happy result!
--Rheanna