It is recommended to have an ultrasound examination on the fifth or sixth day of menstruation to find out the basal follicles, which are relatively small on the fifth and sixth day of menstruation, usually around 0.5cm. As long as the hormone level in the body is normal, the growth rate of the follicle is about 1.5-2mm per day, and the diameter of the follicle can reach 1.0cm around the twelfth day of menstruation, and the follicle can reach dominance on the fourteenth day of menstruation, usually around 18mm. When the follicle reaches about 20mm in diameter, it will rupture and expel the egg, so it is necessary to monitor the basal follicle on the fifth or sixth day of menstruation.