There is no such thing as how long after the earliest coitus can be measured, it is recommended to take a blood test after 10 days of coitus. After intercourse, the sperm and the egg can meet and combine to form a fertilized egg, the fertilized egg generally in 6~7 days or so, after the successful implantation, the trophoblast cells begin to secrete HCG (human chorionic gonadotropin), which will be detected from the blood first, and it will be more accurate to go to the test after 10 days of intercourse, so don’t go to the blood test too early. Each hospital has different testing instruments and reagents, and the reference value after the blood test is also different. A result greater than the reference value, accompanied by a pause in menstruation, indicates that a woman is likely to be pregnant. If the result of the blood test is pregnancy, you can go to the hospital for an ultrasound examination around 35~42 days after menopause to determine whether there are yolk sacs, fetal buds and fetal heart in the uterine cavity.