Three months typically equal to 91 calender days and 91/7 = 13 weeks. However, there is no wall calender hanging inside our body and for immune system, 12 weeks are as good as 13 weeks, so for internal defence mechanism of human body, its the same, no difference.
no idea..test at 13 weeks..just not to have any doubts after a 12 week test..otherwise u wud want to test again..