How long after unprotected sex can i take a pregnancy test?

The placental chemicals that turn a test positive are detectable ~2 weeks after exposure if conception occurred. False negative tests are common if done too early. If the test is positive it is usually a valid test. If you did not ovulate at the needed time, or a conception did not attach inside the uterus, no pregnancy would be expected.

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.