How many weeks pregnet is my wife?

My wifes first day of last period is on 24th March 2011, we were in intercouce for a week on April (Apr 16 to April 24) . Now my wife is pregnent . Now I want to know how many week she is pregnent and the week which I mentioned is safe period. How it happens pregnency in this safe periods.Is there any problom for my baby ?