Follicle monitoring is usually done 3-5 days after menstruation to observe the growth rate of the follicles and whether they are expelled, as well as to observe whether the endometrial growth is synchronized with the follicle growth. There will be multiple follicles developing at the same time at the beginning of each menstrual cycle, but only 1 or 2 follicles will be able to develop to maturity. Small follicles will be found in the ovaries by ultrasound on the 3rd-5th day of menstruation, and on the 5th-7th day of menstruation, the smallest follicle will be 4-5 mm in diameter, and then gradually grow to its maximum size and ovulation will occur on the 14th day of menstruation on average. In addition, the frequency of follicle monitoring should be determined by the size of the follicles. When follicles start to grow after menstruation, women can start monitoring at 2-3 day intervals to observe the follicle growth; while during ovulation, frequent monitoring is needed, even daily, and ovulation usually occurs when the follicle growth reaches 1.8-2.0mm.