A line is a line is a line....HOWEVER...(and here's the not fun part), if any of the major genetics didn't mesh up correctly -which happens all the time- a chemical is secreted by the ova that either doesn't allow implantation, or that breaks the bonds and starts releasing it. The fertility docs I know say that on average, a woman having unprotected sex has between 4-7 of these per year (we could go into the details, but I won't here)...and doesn't even know it...because their period isn't late. Obviously the first 2-3 months are what's commonly known as the "dangerous" period...but it's really the very first 2-3 weeks where the body really checks and makes sure there are an appropriate number of eyes, heads, limbs, the ability to make blood, etc.
So, now that I've been the stick in the mud, can everyone else issue congratulations? Because, K., more then likely #3 is no longer ttc, but on the way. :) If you can stand the suspense wait a week...otherwise go burn through some tests.