It depends on the Ovulation, really. Your period happens exactly 14 days from the day you ovulated. If your period is generally regular, you can figure out the exact week of ovulation and avoid sexual intercourse during that week (Called the "Rhythm Method", though it is not 100 percent.) Pregnancy tests are most accurate at at LEAST 5 days after your missed period.
at 4:52 PM on Jul. 1, 2008