How soon after a missed/late period can a doctor tell if you are pregnant?

Pregnancy ?'s ?: If your periods have been regular (24-32 days apart), and pregnancy is possible, a serum HCG test is accurate as early as the first week after the expected period. If you have been under stress (emotional, heavy exercise) your period may be delayed , as ovulation may have been later than usual.

See below: 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.