Pregnancy can be detected as early as 7-10 days after sexual intercourse and as late as between 35-42 days after conception. There are many different ways to check for pregnancy, and the time to detect pregnancy varies from method to method. The earliest method to detect pregnancy is the blood HCG test, which is usually done 7-10 days after ovulation. A urine HCG test can also detect pregnancy at an earlier stage, usually about 2-3 weeks after intercourse. An ultrasound test can only determine pregnancy by observing the gestational sac, usually about 3-4 weeks after intercourse, and can observe whether the sac is developing normally.