My o.b. said that im fertile today nov.8,08 and she said also it is last for 72 hrs.only is it true?

when i visit my o.b. she do my trans vi and she said that im fertile today nov.8,08. best time to make love bec. we try to have a baby she said also it is last for 72 hrs. only it means until nov. 11 only safe on nov. 12? i want to know if its true.