According to the National Institutes of Health, it is possible to get pregnant if you have not had a period in six months. While not menstruating can indicate infertility, women who have missed many periods should still use contraception if they do not wish to get pregnant.
Women who previously had regular menstrual periods, have missed more than three periods in a row and are not pregnant are considered to have secondary amenorrhea, according to the NIH. Amenorrhea can be caused by a variety of conditions, commonly including polycystic ovarian syndrome and thyroid problems. Amenorrhea can also be caused by excessive exercise, eating disorders and extreme stress.