How accurate is a blood test when testing for pregnancy ?

98-99 percent: False positives are very rare, false negatives can occur if the test is done too early (less than 7 days after conception).

Time of test: 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.