Negative pregnancy test 10 days after anticipated missed period and 20 days after sex. how accurate is this negative?

Accurate : Home pregnancy tests are very accurate. They are the same tests used in a doctors office. They can detect a pregnancy 7-10 days after conception. The best time to take one is when you miss your period. If you doubt the result you can repeat it in 1-2 days. If you were pregnant the hormone level would rise enough for detection.

If properly done: A negative pregnancy test means that in all likelihood there is no pregnancy. 10 days after and anticipated missed period a transvaginal ultrasound would likely demonstrate a pregnancy in the uterus.