I get the feeling that this is Shaw's (?) offspring ?.
One of the crew becomes infected in some way, rapes Shaw (the guy sat on the bed holding his head in his hands, with her body on the bed next to him), she later stumbles into the medical bay covered in blood and collapses. A short while later her stomach rapidly expands (with her in terrible pain), then she gives 'birth' to this.
Very 'perverted' to quote Ridley Scott ?.