Ob/ gyn question generally speaking, how many days after sex can a blood pregnancy test pick up pregnancy for 100% accuracy? (my cycle is about 40 days long??

Generally speaking, at fourteen days PRIOR to the onset of a period, ovulation occurs, so allowing for late implantation, wait until 5 days after your period is due, and a serum HCG test will accurately detect pregnancy.

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.