If the menstrual cycle is 30 days, it is counted from the first day of menstruation, about 16 days is the time of ovulation, then according to the time of ovulation and then count forward 4 days, 3 days after this is the ovulation, so it is recommended to calculate according to their own menstrual cycle, basically in the next menstrual period 14 days before the day of ovulation, so according to the day of ovulation is very easy to calculate the period of ovulation. In addition, if the menstrual cycle is irregular, it is difficult to calculate the time of ovulation through the menstrual cycle, and it is also difficult to deduce the ovulation period, so it is recommended to go to the hospital in this case through the ultrasound to check, to understand the size of the follicle. If the follicle basically reaches dominance then it will ovulate every other day, so it is necessary to do ultrasound examination.