Some women claim that they just have that "gut feeling" that they are pregnant and they turn out to actually be pregnant.
The problem with this is many women have that same gut feeling because they are worried about being pregnant when they really aren't. The mind is a very powerful thing that, if stressing about it enough, will begin to play tricks on you. You can begin to experience those pregnancy symptoms because of the stress even though you aren't pregnant, such as feeling bloated, nauseated, or cravings. All of those are symptoms that can be influenced by state of mine instead of true "scientific" symptoms that can't be altered by mindful thinking.