From what my doc told me, a chemical pregnancy is when the egg gets fertilised but for whatever reason it is not a viable pregnancy and you gennerally lose it without even realising you were pregnant in the first place. This happens to about half of all pregnancies. The most common belief in the medical community about it is that the DNA strands don't fit together right and so AF comes to take away the "mess" or at least that is what my doc told me on my last visit. AF in these cases are usually earlier or later than we expect them to be but some are still right on time. It is often possible to get a + hpt before the baby is gone and AF arrives.