It is also possible that the follicles are not seen on negative ultrasound and cannot be seen on ultrasound after ovulation. On days 3-5 of the natural menstrual cycle, several sinus follicles of 2-9 mm are visible in the ovary. As the follicles develop, eventually there is only one dominant follicle and the rest of the follicles are successively atretic. The follicles are visible on the preovulatory examination by negative ultrasound, starting with several small follicles, eventually only one dominant follicle is present at the time of ovulation, and the follicles are not visible after ovulation until the start of the next menstrual cycle. In a normal natural cycle, follicles go through several stages of growth and development, maturation, ovulation, corpus luteum formation and corpus luteum degeneration. The growth and development of the follicle can be clearly observed and accurately measured by ultrasound, which can accurately determine whether the ovary is ovulating normally and thus guide coitus or assisted reproduction.