If you have a regular menstrual cycle and have your period on the 4th, you will ovulate around the 15th-24th according to your 30-day cycle, which varies from person to person. Ovulation usually occurs on the 14th day of the next menstrual period, and the 5 days before and after this day belong to the ovulation period. If you usually have irregular periods and cannot predict the time of your next period, you cannot determine ovulation based on your menstrual cycle, and this method is only applicable to women who have regular periods. If the menstrual cycle is early, ovulation will also be early; if the menstrual cycle is prolonged, ovulation will also be delayed backwards. If you usually have irregular periods, you need to go to the hospital for an ultrasound examination to measure ovulation, the results are more accurate, and you cannot project ovulation based on your menstrual cycle. Ovulation usually lasts about 10 days, not absolutely on a certain day, and can fluctuate due to various factors such as emotions, fatigue, insomnia, diet and illness.