Warning: blah blah blah, spoilers, blah blah blah.
I would not call myself a horror film fan; I haven't seen many and don't really find them that scary. My inner cynic comes out when watching the jumpy ones; the longer the silence lasts, the likelier a 'scare' is going to happen. The horror films I have enjoyed are not simply out to scare you; they deal with other issues like family, and our relationships with the 'other.' Or they are campy fun!
Roman Polanski's Rosemary's Baby (1968) and Ridley Scott's Alien (1979) are intelligent horror films that have very different approaches to the experience of pregnancy and birth. Based on Ira Levin's novel, Rosemary's Baby tells the story of Rosemary Woodhouse and the strange goings-on that happen while she is pregnant with her first child. At the time of conception she seems to have a dream that the devil rapes her as she is surrounded by her neighbours and her husband. Rosemary famously declares 'This isn't a dream! This is really happening!', but her husband dismisses the scratches on her body as a consequence of rough sex the night before. The growing foetus makes her very sick, but when Rosemary believes her neighbours are trying to harm her child she is very protective. The birth is as obscured as the conception, and Rosemary is kept in the dark about the whereabouts of her baby. The last scenes reveals two twists: one, that baby is the antichrist, and two, Rosemary loves it anyway.
Scott's Alien is both science fiction and horror. It could easily be a group of people stranded in an isolated cabin dealing with a pesky intruder; its being set in space adds another layer to the story. The characters are as isolated as humans can get in the universe, travelling on a ship that is far from earth. A team aboard the Nostromo, which is heading back to earth after a mission, receive a distress call from a nearby planet. One of the crew, Kane, who go exploring the planet is attacked by a strange creature that comes out of an egg. It remains attached to his face; it does not kill him but uses his body as a host (the crew is not sure what for). It eventually drops off and Kane wakes up with no memory of what has happened. In the of the most famous scenes in film history, an alien bursts out of his stomach at mealtime. It proceeds to terrorise, and kill, all crew members on the ship, save Ripley who escapes via a shuttle.
Pregnancy has been used as a dramatic point in every genre of film. What makes horror different from others is its emphasis on and exaggeration of the violence of this natural experience. Rosemary's body is taken over by her growing child; during the first few months Rosemary grows considerably thinner, and she is struck with almost unbearable pain. This is comparable to the alien's attaching itself to Kane; he is acting as a host for the alien. It is not in its interest to kill him, but it renders him completely immobile and unconscious. They can't separate the alien from Kane without killing him. Both Kane and Rosemary are acting as hosts to these dependent creatures.
Of course, these 'creatures' are not your typical foetus. Kane's alien baby is organic looking but clearly not human. Beyond him acting as a host Kane means nothing to the alien; there is no 'love' between them. It kills Kane when it is born. With Rosemary and her baby, the normal mother-baby bond is less straightforward than usual: the child is the Devil's Son. The people around Rosemary also view her as a host for the child. Minnie Castevet, neighbour and devotee of the Devil, keeps giving Rosemary tonics to help with the baby's growth. Though throughout the film people appear to be caring for Rosemary, everyone is actually focused on the unborn baby. Strangely enough, Rosemary is as well, even when it is causing her great pain.
One of the most striking scenes in Rosemary's Baby is a moment after a party Rosemary and her husband have hosted at their flat. For the last few weeks Rosemary has looked sickly: pale, she has lost weight, and she is crippled through pain. To top it all off, the foetus is not moving. After the party she fights with her husband about getting a second opinion about her pregnancy. Suddenly the pain stops. Then Rosemary starts smiling. 'It's alive!' The foetus is moving, and she is far happier about that then about being free from pain. There is something both understandable and yet highly uncomfortable about this response. While she is full of joy at the life inside her, Rosemary still looks like death with her gaunt, pale face. At what cost is this child being kept alive?
It is interesting to note the presence of rape in both these pregnancies. Rosemary has a dream where she is raped by a monstrous figure, watched by her husband and neighbours. She wakes with scratches on her body. Her husband explains it by saying that, despite Rosemary being unconscious, he didn't want to miss a chance for them to get pregnant, and got a bit carried away. She may have wanted to get pregnant, but Rosemary is deprived of her ability to choose when (and with whom).
Kane clearly doesn't have any choice in what happens to him. The alien attacks him, and latches on to his face. His face is obscured by the creature; it takes away his identity, as well as only keeping him alive to keep itself alive.
One could question the behaviour of Kane, touching a strange object that looks like an egg while exploring an alien planet. But curiosity is a human trait, and without it the film (and many others) would not last beyond Act One.
These rapes are not performed because of unbridled sexual desire or as a display of power over a character, the common reasons for rape. Instead they are objectified as hosts, seen as simply a place to live in for a period of time. It is notable that in Alien it is a man that is raped and impregnated; Dan O'Bannon (screenwriter) argued that it was intended as 'payback' for all the women in horror who preyed upon by male creatures.* You could argue that this is a display of power by the alien, but it does not specifically target Kane, only latching onto him because he is there. Likewise Minnie and her husband and their friends use Rosemary because she is close by. This adds to the horror of the situation; they are not special, just convenient objects for others to use.
The pregnancies in Rosemary's Baby and Alien are animalistic in their violence, treating the 'mother' as nothing more than a host. There is no love exchanged, though Rosemary does love and care for her child; it being the Devil, it is unlikely to love her back. Of course, normal pregnancies are (hopefully) less violence. However, the writers and filmmakers are cleverly exaggerating aspects of human pregnancies and birth. The outcome is usually happier, as one doesn't have an alien kill you, or give birth to Satan's Son, but there are uncomfortable, painful, even traumatic occasions in even the most normal of pregnancies. Unlike horror films about being possessed by spirits, or body horror that explores transformations (werewolves or vampires), these two films play upon a very real experience. It is this that makes them so unsettling.
*Mark Kermode, http://www.theguardian.com/film/2003/oct/19/features.review