Pretty much most can be chalked up to wishful thinking and TTC obsessing plus the usual progesterone production that starts as of ovulation... signs of increased progesterone, which is normal in your luteal phase whether you are pregnant or not, include tender breasts... fatigue... bloating... and mood swings... all of which can both be a pregnancy sign AND just a sign that ovulation has happened, even if pregnancy was not achieved (which would explain why non-pregnant women can convince themselves they are pregnant.)
I do not see too many women who claim early pregnancy (pre-implantation) signs that could not be chalked up to progesterone.
I do see a LOT of women who find these signs either only one cycle out of a ton charting or every single cycle and not end up pregnant, even though they were convinced they knew of fertilization the moment it happened (lots of women know the moment it happens, unfortunately many of them are wrong)
now for me, this pregnancy... the only two symptoms I had before I felt implantation/saw it on my chart (and even THAT could have been a day off making my 5dpo symptom happen right at implantation) is that at 2dpo I had spotting. I never ever have spotting. I do not think it was implantation but rather post-ovulation spotting (it was brown... old blood. not really a pregnancy symptom but was worth noting as I had not had it before!) and at 5dpo I started feeling nauseous... which could be chalked up to two things... possibly implanting at 5dpo and feeling the effect right away... or that I had overate at Christmas (didn't seem like it, but was possible) and coincidentally as soon as that nausea wore off I was into my pregnancy nausea.