Although this is one of Roger Corman's "ALIEN" inspired movie it's different enough to be interesting.
A group of scientists are hiding in a facility below the Mojave desert. It's years after "The Accident", which killed off the majority of the human population and transformed the survivors into creatures they call "Gargoyles" because that's what they look like. A scouting party is killed above ground and then another is sent out to find out what happened to them. They were killed--but they also find a cave of dead bodies, human survivors. They also find a young woman who is terrified and bring her back. Tests show that she is pregnant. But she's pregnant with a Gargoyle child and it is born within a few days. It escapes into the ducts of the facility and then grows to full size where it starts to kill everyone.
Andrew Stevens is the main character, David, and Starr Andreef (DANCE OF THE DAMNED) is his girlfriend, Sue. Terri Treas (ALIENATION: THE SERIES) is the biologist and George Kennedy is the leader, Hal.