Hello, I have gone through the query and understood your concern. Blood test for pregnancy can read positive as early as one week after ovulation if conception occurs. If she was ovulating at the time of the intercourse, conception chances are high and if the test is done at least a week away, the result would be mostly positive. The blood test moreover, is very sensitive and specific and chances of error are very less. You can further confirm with a trans-vaginal sonogram done a few days after the expected period. Please remember that ejaculation is not necessary to cause conception; the pre-ejaculate is enough. Hope you find this information useful. Take care.