I found this really interesting:
The early pregnancy factor
The early pregnancy factor or EPF is a imuno-suppressant protein and has been detected in mammals right after conception. Its function is to prevent the immune system from attacking the new embryo. Research carried out on human embryos showed positive results within 48 hrs after conception.
Pregnancy symptoms 48 hrs after conception?
Some women just know right away when they are pregnant. Also, a large percentage of women start to feel pregnancy symptoms very early on, even as early as a few days after conception. Health professionals will tell you that this is not possible since implantation hasn't occurred yet. However, in light of the research done on the early pregnancy factor, it could indeed be possible! The imuno-suppressant functions of the EPF could cause pregnancy symptoms to occur especially if your body is more sensitive to this protein. Morning sickness, in particular, has been linked to immunity suppression