Can you take a pregnancy test 3 weeks after sex and a week before period is due?

Yes, you can: Home urine pregnancy tests become positive as early as a week after conception (sperm meeting an egg). If a pregnancy test is negative, and a woman is pregnant, another test a week from the previous one will likely be positive. If a woman is pregnant, at the time of her anticipated menstrual period the test will be positive.

May not be accurate: The home pregnancy test may not accurately reflect the correct result as it may be too soon.