My friend had unprotected sex New Year's Eve and hasn't started her period since. She was supposed to start January 3, so she took four pregnancy tests (bought from pharmacy) on the 27 and 28 of January. All of the tests said that she was not pregnant -- how accurate, considering the day her period was missed and the days she took the tests, are these tests?
Her period usually starts on the 3 of each month, so a month without starting is highly unlikely. So my next question: if she's not pregnant, what could be causing this?