Let's start with Alien. When you consider the involvement of Swiss conceptual artist H.R. Giger, it comes as no surprise that this is a heavily sexual film, starting with the phallic design of the xenomorph. In particular to maternity, though, the most striking thing is the set design inside the Nostromo. The tight, claustrophobic hallways clearly invoke the birth canal, presenting it as a horrible, cold place. Given the way the creature arrives in the ship (the facehugger itself mines terror from the fear of male rape), it's fitting: the xenomorph is, in a perverted way, this crew's "child," and it's threatening to destroy them at every turn. And it strands Ripley (Sigourney Weaver) and company without help; their computer system, telling named "Mother," cannot compute their requests for protocol to eliminate the monster.
Of course, it wouldn't be until the film's James Cameron-directed sequel, Aliens, that the motherhood theme would become more overt (of course, it helps that Ripley is front-and-center in that film). Yet in Alien, the fear comes from birth itself, and - bear with me - the idea that the child, a foreign body grown within another body, could be some sort of aberration.
Mama, on the other hand, takes a different approach. The two girls - Victoria (Megan Charpentier) and Lilly (Isabelle Nelisse) - lose their biological mother and are raised in a cabin deep in the Pacific Northwest. The film's main source of tension comes from the competing adopted "mothers" that step in to take care of them: Annabelle (Jessica Chastain), the girlfriend of the girl's surviving uncle (played by Nikolaj Coster-Waldau), and "Mama," the spirit that protected them in the woods. The film relies a tad too heavily on jump-scares and the fast-moving ghosts borrowed from Asian horror to elevate the film to classic status, yet it derives terrific conflict from the fears of adoptive motherhood. These children have already been through severe trauma - even without the ghost. Annabelle's fears stem from the fear of screwing them up even worse, or, at the very least, not being able to help them recover.
There are, of course, a ton of horror movies out there that utilize maternal fears for their terrors. But Alien and Mama, an unlikely pairing, present two different kinds of fears in interesting ways.