Lack of menstruation is the first, albeit uncertain, sign of early pregnancy. There are other conditions besides pregnancy which might also cause lack of a period. These conditions include: persistence of a functionally active ovarian cyst; hypogonadism; PCOS; coming off birth control pills, lack of ovulation during menstrual cycle for any reason, etc. Negative pregnancy tests are fairly secure signs that you are not pregnant. You can visit your gynecologist to identify the reason why you aren't getting a period and are, at the same time, not pregnant.