How soon can you be pregnant after sex?

It takes a couple days for sperm to reach the egg in the Fallopian tubes. It takes a few more for them to join and form the initial conceptus. This must then travel to the uterus and implant. By 7-10 days you can have this complex seated and begin to create the hormones that turn a pregnancy test positive. By 2-3 weeks there is enough hormone to be detected. Some conceptions never implant.

Conception occurs within day or so of the sex. It takes about 3 weeks for the embryo to make enough HCG for a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. If the result is negative repeat the test in 4-5 days. Use first morning urine and follow instructions for the test carefully.