My understanding of a biochem is a form of nature's quality control. It is a pregnancy that is detected by biochemical means only. (I think?) It is a very early pregnancy loss. But a real loss still, which I think is lost on a lot of people. Especially if the mom/parents were hope to conceive.
I think that most go undetected, as in, "Geez, my period was late and has been weird." But for those who are being closely monitored it is caught and understood as a biochem preg.
Hope that helps.