If you have not had a period for three months and you are not pregnant, it may be due to the following reasons: 1. If you have had a history of uterine surgery before three months, you must rule out that the amenorrhea is caused by uterine adhesions. 2. Long-term use of short-acting contraceptives may also lead to amenorrhea. 3. 4. If you have not had a period for three months, it may be caused by polycystic ovary syndrome, which can be initially diagnosed through ultrasound and sex hormone tests.