In general, ultrasound is appropriate on 3-7 days after menstruation.1. Ultrasound examination on 3-7 days after menstruation can clearly determine whether there are lesions in the endometrium and exclude the possibility of endometrial polyps.2. Physiological cysts of the ovaries can be excluded, and sometimes it is necessary to monitor whether there is ovulation, and vaginal ultrasound examination should be performed around the tenth day of menstruation, with an interval of 2 or 3 days depending on the results of the examination. Repeat the examination. 3. If it is necessary to determine whether there is ovulation, to determine the thickness of the endometrium and to decide whether the decrease in menstrual flow is due to endometrial factors, ultrasound examination is needed before menstruation, and the time chosen for the examination is different for different examination purposes.