Blood HCG can be checked about 7-10 days after intercourse. Sperm and egg combine about 30 hours after fertilization, and the fertilized egg moves toward the uterine cavity with the help of tubal peristalsis and tubal epithelial cilia, and the embryo implants into the uterine lining about 6-7 days after fertilization, which is the time of implantation. Soon after the fertilization of the egg can be measured by radioimmunoassay, the HCG in the blood of the examinee is elevated, which can be proved to be pregnant. Clinically, you can also use early pregnancy test paper to test the urine of the test subject to observe whether pregnancy, but urine HCG is not as sensitive as blood HCG, usually need to wait until a week after the menopause can be detected. Positive urine HCG and blood HCG can only diagnose pregnancy, but an ultrasound is needed to determine if the pregnancy is intrauterine. The main purpose of ultrasound is to determine intrauterine pregnancy and to rule out ectopic pregnancy and trophoblastic disease, among others.