The earliest pregnancy can be detected should be on the 10th day after making love. This is because fertilization occurs within 12 hours after ovulation, and on the 6th day after fertilization, the trophoblast cells begin to secrete traces of chorionic gonadotropin, which can be measured in the serum of the pregnant woman on the 10th day after fertilization and can be used to diagnose early pregnancy. Therefore, if coitus coincides with ovulation, and it happens to be within 12 hours after ovulation, and fertilization is successful, chorionic gonadotropin should be detected in the maternal serum on the 10th day after coitus. If the diagnosis is further confirmed, a gestational sac should be visible on ultrasound 3 weeks after fertilization.