From the Montreal Mirror:
It was Ridley Scott who, with his second feature film, Alien (1979), brought us one of the greatest gender-busters in cinematic history, Ripley. In Dan O'Bannon's original screenplay, every character in the film was named, but not gendered. Thus it was up to Scott to pick and choose which characters would be male or female. The result was some of the most subversive casting imaginable, running counter to every gender stereotype possible. Ripley, the strongest, most resilient character, went to Sigourney Weaver. And the character who is impregnated with the alien was played by a man, John Hurt.Surprisingly, Scott claims his casting of Weaver in the title role held little significance for him at the time. "Once we got into it, I got so used to Sigourney in the part that I never really separated it from her performance, other than her being the survivor. It was kind of a neat idea, to make what could have been a male character into a female one. It was just kind of curious."
And from AwwwGeez:
Ripley, second officer of the freighter Nostromo and portrayed by actress Sigourney Weaver, is presented to the viewer as the strong female character in Alien, a tough, no-bullshit type, especially when compared to the other female characters in the film. Lambert, the navigator for the Nostromo, is high-strung, panicky and hysterical---in other words, she embodies the typical movie-female response to fear. Ripley, on the other hand, doesn’t buckle or freeze under pressure as Lambert does when faced with the Alien: instead, her fight-or-flight instinct is highly honed, and she reacts as calmly as possible under the circumstances. The presence of a typically out-of-control woman character serves to point up this difference: compared to Lambert, Ripley is a rock. As a result of this portrayal, and its novelty, the viewer (typically male and typically 18-35) is already thinking in terms of Ripley being “different” in some way, and difference then becomes one thing she has in common with her enemy, the Alien itself (Torry 352). In this way, the apparent feminist agenda of the film is already being undermined.
Here is the heroine of the Alien film series being threatened by one of the creatures. Note that Ripley carries another smaller alien within her -- soon to become a chest burster.