Alien popularised it, taking the most natural thing in the world, motherhood, and making it the subject of horror. The reason? Well, Hollywood is controlled by men, and this is a very masculine fear.
The most famous scene in Alien is the infamous "chest-buster", where John Hurt is impregnated by an alien foetus which emerges from his chest and causes untold horror. So much of the female experience is presented here: the that of rape, the pain of childbirth, and the (thankfully less common now) potential for death during childbirth. These are facts that many women have to live with, but by making a man the victim, everything is presented in a grotesque, horrifying way. This is not to say that Alien isn't feminist, in fact, it gives us one of the greatest female heroes of all time in Sigourney Weaver's Ripley, but it does play with the patriarchal societal norms to generate it's horror. And significantly, the only survivor of the whole ordeal is Ripley, a woman.
So that's childbirth, but what about when the pregnancy itself becomes the source of horror? Look no further than Rosemary's Baby. As much as I loathe to offer Roman Polanski praise, Rosemary's Baby is perfectly unsettling as the titular character, played by Mia Farrow, is raped by a demon and forced to carry the spawn of Satan. Rosemary's pregnancy takes a very extreme toll on her mental and physical state, but where it differs from Alien is that Rosemary is a woman. Alien's horror came from placing a man in an exaggerated and painful human experience. Rosemary's Baby confronts, via metaphor, the very real experience of a woman becoming pregnant via a rape. The fact that this does still happen in our society is what makes Rosemary's Baby so terrifying.
Carrie showcases a similar fear, but while Mrs Bates was a psychological presence, Carrie's mother is very literal. Carrie, played by Sissy Spacek, is abused by her religious fanatic mother Margaret (Piper Laurie), which is one of the factors triggering Carrie's telekinesis. When her powers are revealed, Margaret believes her to be a witch created the night Carrie's father raped her, leading to the film's traffic climax. Like Rosemary's Baby, Carrie is a child of rape, an unfortunate fact of womanhood and a rich source of psychological horror.
There are plenty of other examples of both mothers and their offspring generating horror. Jason's mother in Friday 13th and Damien in The Omen being just two of them. Childbirth and motherhood is so natural, the reason for life on earth, that even just manipulating the tiniest thing can make the relationship between mother and child unnatural and disturbing. Horror as a gene takes this to the logical extreme, using extreme metaphors to highlight a deep-seated fear in all of us:
what if we can't trust the person closest to us?