Uh, not sure how your logic is working there.The only person that we can speculate from is Skye's mom. She had abilities but gave birth to a human who needed terragen to gain powers. Perhaps, in the womb, the child is protected from terragenesis. (Also that Bahrain incident was about an active inhuman using crystals to activate her inactive daughter).
Terragenesis is more of a catalyst effect, not a permanent one. You get exposed to the mist, it triggers a chain reaction awakening your dormant genes/abilities. Your body doesn't retain the terrigen mist afterwards. If it did, a simple blood transfusion is all any Inhuman would need to transform other latent Inhumans.
Considering Skye's mother was transformed loooong before Skye was conceived, it doesn't prove anything at all.