My gut tells me the xenomorph just dies in a stillbirth. Facehuggers don’t implant eggs, they deposit a tumor-like growth into their victims that grows a xenomorph. Even when a chestburster is surgically removed, the host still dies from the (literal) alien cancer growing and sucking up all the body’s nutrients (like normal cancer). Necromorphs both are made up of dead tissue and already use their entire metabolism to fuel the transformation into the various creatures, so there’s literal fuel for the cancer to draw upon and what is there will likely kill it anyway.
But then I remember that the Feeder necromorph
are still living people who ate necromorph biomass and suffered a slow transformation. And the Alien books introduced the - ahem
, a xenomorph that gestated inside a person suffering from necrosis. The NecroXenomorph inherited and adapted the degeneration into itself.
So yeah while the most likely answer is that the gestating chestburster just straight up dies inside a necromorph, there is some vague chicanery in both series that opens the door to a possible hybrid. Though I’d say this hypothetical hybrid would be under the Brethen Moon’s thrall instead of a part of a Queen’s hive.