After taking birth control i only get my period every other month, when would be the best time to take a pregnancy test after unprotected sex?

14 days later: It generally takes 14 days after conception for a home test to turn positive.

3 weeks: 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