If you have a irregular period because of nexplanon how long after sex will a pregnancy test be accurate is 21 days after long enough?

Yes, A pregnancy test 21 days after sexual intercourse should be accurate whether or not nexplanon is used for the period is irregular.

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.