The Terror Within (1989)

It's the post-apocalypse, and the world has been changed by "the accident", a chemical warfare experiment gone awry. At an isolated subterranean complex, a group of people survived because they were able to get the antidote for the illness. They rescue a surface survivor from the gargoyles, who unfortunately had time to impregnate her, and when the "baby" is born and escapes into the ventilation ducts, they begin experiencing an attrition problem.