Women’s menstruation can be affected by many factors, as long as the menstruation is not more than a week ahead or delayed, it is a normal physiological phenomenon, menstrual changes in the day of ovulation will also change accordingly. Whether or not ovulation occurs on the 12th day of a woman’s menstruation depends on the specific situation. If a woman’s menstrual cycle has been very regular, and each month is 28-30 days, the day of ovulation should be around the 14th day before the next menstrual period. The 12th day of menstruation should belong to the range of ovulation, the probability of ovulation is relatively large, some women due to endocrine disorders menstrual cycle is not regular, the 12th day of menstruation is not sure whether ovulation, but most of them ovulate during the period of ovulation, and sometimes there will be accidental ovulation. As long as it is not the menstrual period, any other time there may be ovulation, the safe period is not absolutely safe, if you do not intend to have a child must do a good job of contraceptive measures to avoid unwanted pregnancy.