Ovulation is usually not counted from the first day of menstruation. For people with regular periods, it should be about 14 days before the next period, which is almost the middle of menstruation. After ovulation, the luteal phase is relatively stable, usually 12-16 days, most of them are about 14 days. Therefore, it is relatively accurate to push the ovulation period about 14 days forward from the next menstrual period, rather than how many days back from the first day of the first period. Of course, for those who are able to do so, you can use ultrasound to monitor ovulation, which is more accurate because ultrasound can see the size of the follicles and the process of growth and development. Usually the follicle will ovulate at around 1.8-2cm. You can also measure the urinary LH level at this time, if it is high, you will ovulate, while if it is low, you may need an ovulation promotion injection.