If you usually have a regular menstrual cycle, once every thirty days, it is recommended to start follicle monitoring by ultrasound on the twelfth day of your period, preferably in the afternoon. If you find that the follicles have reached dominance, or if the follicles are about 20mm in diameter, then you can choose to have intercourse, which will increase your chances of pregnancy. This is because the sperm can survive for a relatively long time after entering the vagina, about 2-3 days. If the egg is ovulated, the survival time is about 24 hours, so a reasonable timing of intercourse will increase the chances of pregnancy. Therefore, if you have an ultrasound in the afternoon and find that the follicle has reached 20mm in diameter, you can have intercourse the same evening and your chances of getting pregnant will be greatly increased.