Birthing the chesburster

Leading from:  
Early Development of the Alien Script

a) Ron Shusett had the idea of a man face-raped by an alien organism later giving birth to an alien baby, and then Dan mulled over the whole idea, he wondered about how the baby alien creature would come out, he had thoughts about them coming out from various orifices and so on and soon himself woke up in the middle of the night with a realisation of how it should be. He got up off the couch and went to Ron's bedroon and told him his idea. He said to Ron, "The monster bursts out of his stomach" and he went on to explain that the alien not only lays its egg inside the man, but grows inside and emerges as a baby, splattering flesh and blood everywhere.

b) The way he had it was having the creature burst out of the victim's stomach rather than actually the chest,  seemed to reflect very much the stomach and bowel problems he suffered from that would eventually be diagnosed as Crohns disease which would eventually kill him in 2009. (See: Medical-inspiration")

c) The actual idea though for a creature that burst out of a victim's body had been germinating in him for some time. The initial concept came to him when he saw the comic book issue No.8 of the comic book Weird Science originally published in 1951 and then Tim Boxell's comic book story "Defiled" from Death Rattle #1 (1972) which certainly help to shape the idea,  and there would be experiences that he had with his own intestinal issues (See Medical Inspiration) that he was dealing with that made the idea of a what would be known as the Chestburster even more of a personal experience 

Tim Boxell's "Defiled" from Death Rattle #1 (1972)

d) Dan was now so sure that they had cracked it and had got what they needed to write the whole movie, since no one had ever seen anything like that on the screen and in three weeks they were able to write their whole story.

Although Dan had talked briefly in the past about how the alien chestburster came from his experience with stomach problems, the details about how he arrived at the idea came to the surface after soon he died.

Issue No.8 of Weird Science

Source quotes 

1. Pat Lowry: "He... he was explaining to me that, erm, he thought and thought and thought he written, virtually written Alien, but he couldn't think, he was saying, couldn't think really how to bring the alien into the picture, in other words, the John Hurt bit, erm, the, erm, alien bursting out of the body, he said, and he said " I was trying to think how I was going to do it", he said "I had thoughts of them coming out in various orifices and that sort of thing," he said, "and suddenly in the middle of the night, I woke up and thought, yes, I know how it's going to happen, that's how it's going to happen, " he said "and that's how it was" Yeah, so obviously it didn't start with that coming out and work the story around it, he worked the story out and that came afterwards, you know. (Pat Lowry worked as a Unit Driver for the Alien production and  chauffering Ridley Scott' during Alien , interviewed in Vennis Lowe's documentary Alien Makers 5)
2. Dan O'Bannon : As soon as I heard it, I thought, oh well, that, that's it. That was one of the ideas that made it possible to uhm, make this thing worth doing at all. This is a movie of alien interspecies rape, that's it, that's scary, that scaring because it hits all of our buttons, all of our unresolved feelings about sexuality, all of them. (Alien Evolution documentary 5:42)
3. Ron Shusett: .... then I said well "Dan, what happens by this way, you'll have a human giving birth to an alien" and the first thought was that we'll operate on him and take him out and Dan said "No, No, what happens is that he doesn't have to operate, in the middle of the movie he comes bursting out of his chest", and we both looked at each other in mutual horror of what we created and right then we knew the whole script would be finished and it would be everything we wanted it to be, that among everything, a sort of collaboration between once I came up with impregnating and I said to Dan, no one, you ask how he gets on board and nobody's seen a moment like that in science fiction history Dan, we both held our mi... and that turned out to be the making of our careers that moment, in three months from that moment we had the script ready and it all lived up to that first twenty nine pages. (Ask Mr Kern show October 27th 2012 Part1 , 17mins in,. http://www.askmrkern.com, )  (see partial transcript from the Ask Mr Kern epsiode, relating to invention of the lifecycle) 
4. Jason Zinoman: O'Bannon mulled over the idea when he went to sleep and woke up in the middle of the night. He walked over to Shusett's bedroom and proposed what would become the most shattering horror scene since a bewigged Norman Bates pulled open the shower curtains. "The Monster "O'Bannon told his friend " bursts out of his stomach. ". 
   (see Shock Value, Chapter 10 Stomaching It, p179 to 181)
5. Jason Zinoman: O'Bannon went on to explain that the alien not only lays its egg inside the man, but grows inside and emerges as a baby, splattering flesh and blood everywhere. The idea of an alien violently bursting out of a stomach was of course very personal for O'Bannon, that's what his sickness felt like. He could also see that this would be a potent metaphor for childbirth. Increased access to birth control pills and the rise of abortion as a political issue in the seventies made reproduction a source of much paranoia.
   (see Shock Value, Chapter 10 Stomaching It, p179 to 181)