There are many reasons why menstruation is delayed for 12 days, as follows: 1. Pregnancy: When sex is not done with contraceptive measures, first of all, we must exclude accidental pregnancy. 12 days after the last intercourse can generally be tested with a pregnancy test or early pregnancy test paper. 2. Endocrine disorders: If pregnancy is completely excluded, it is likely that endocrine disorders lead to delayed menstruation. If a woman stays up late, sleeps late, has bad moods, changes her environment, or is under a lot of mental stress, it may affect ovulation and lead to delayed menstruation or amenorrhea. 3, Drugs: If a woman has recently taken drugs that affect menstruation, such as emergency contraceptives, diet pills, or antidepressants, these may also affect ovulation and lead to irregular menstruation. 4, Dieting: If a woman has recently gone on a diet, it may affect ovulation and lead to irregular menstruation. 5. Surgery: If a woman has recently undergone abortion, curettage surgery, etc., which damages the basal layer of the uterus, or produces cervical adhesions, cervical canal adhesions, etc., it may lead to delayed menstruation.