Editing Half-Dwarf (section)

== Alternate Explanation ==
Dwarfs are known for their endurance against poisons, magic and illnesses. This is actually caused by their very aggressive immunological system which attacks anything not-dwarf. Had your world have medicine advanced enough, you'd learn dwarfs cannot receive a blood or organ donation from any other race, and transfusing their blood (even a small amount) to non-dwarfs triggers very strong immune response in the recipient, most of the time turning into an auto-immune response within hours.

The identification gene is recessive, which means that unless both parents have it, the gene will be dormant in their child. If a female dwarf has intercourse with member of any other race, the egg is very quickly targeted by the immune system, and has no chance of settling. Having sex with a dwarf woman on her period ends in a rather nasty rash, though some charlatans advise such acts as a cure for certain enduring STD's (it doesn't work btw, the blood merely aggravates the skin).

A male dwarf impregnating an elf, human or orc woman might very well end with an impregnated egg, but around 2nd to 3rd week the fetus starts attacking it's host, which triggers an immune response from the mother, thus terminating the pregnancy.  These don't even register as a pregnancy, just a brief pregnancy scare due to late period.

There have been attempts at nurturing such conceptions, by suppressing the mother's immune system during pregnancy, but all attempts ended prematurely when the mother invariably got ill from having a bombed-out immune system. Interestingly, the half-dwarf fetus always managed to shield itself from whatever infection threatened the mother's health.  Attempts at exchanging the otherwise-healthy fetus to a womb of a surrogate mother woman in similar stage of pregnancy were attempted by a lich wizard, but any detailed data was lost in the inevitable looting and sacking by heroic adventurers.