Is a blood pregnancy test accurate after waiting 1 week after sex?

Early: This may be too early but often it can be positive. If it is negative, I would check another test in one week.

3 weeks after sex: 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.