A 21mm x 19mm follicle is not round, but oval or close to round. Normally, after a woman’s menstrual period, the follicle will gradually develop and reach 18mm or more at the time of ovulation. So clinically speaking, if ultrasound can find follicles larger than 18mm on the ovary, then it is a mature follicle. The shape of a woman’s follicle is mainly related to the amount of follicular fluid and the follicular tension. A woman’s follicle contains follicular fluid and an egg, but the egg is so small that it cannot be observed by ultrasound or with the naked eye, only the follicular fluid inside the follicle can be seen. a follicle that is 21mm x 19mm is oval in shape and is a mature follicle. Regardless of the shape of the follicle, as long as there is good tension, the follicle is already a mature egg and is completely ready to be discharged. The chances of conception increase significantly if intercourse is performed within a period of 1-2 days.