If you have intercourse during ovulation and the sperm and egg unite successfully, you can detect it at the earliest, about eight to ten days after intercourse. After the sperm breaks through the zona pellucida of the oocyte, it takes about one day to unite with the egg, three days for the fertilized egg to enter the fallopian tube, one day to enter the uterine cavity and implant in the endometrium, and one to two days for the embryonic follicle to settle in the uterus and gradually form the placenta and secrete the human chorionic gonadotropin. It is recommended to check again after two to three days. For many women who have irregular periods or who have intercourse during non-ovulatory periods, it is best to test 3-7 days after the menstrual period to determine if they are pregnant.