Clinically, if ovulation is monitored by ultrasound, intercourse is possible when the follicle diameter reaches 18mm. The average follicle growth rate is 2mm per day, basically the follicle will rupture and ovulate when it reaches 20mm in diameter, so when the follicle diameter is 18mm, it is the dominant follicle and the chance of pregnancy is high. When the follicle ruptures and expels the egg, the umbrella end of the fallopian tube will catch the egg within 10 minutes and send it to the fallopian tube, where it will unite with the waiting sperm to form a pregnancy.