Can you get implantation bleeding three weeks after sex?

Not in real life: In real life, that does not happen. Implantion of a baby embryo happens 1-2 weeks after sex, or 1-1.5 weeks after conception. However, some vaginal bleeding or spotting can happen during pregnancy (no need to call it implantation bleeding). That means a woman should check a home pregnancy test 3 weeks after sex if she thinks she might be pregnant. If pregnant and bleeding, she can call her doctor.