Alien & Aliens
Alien 3 & Alien Resurrection
Prometheus & Alien: Covenant

Alien (1979)

Starring Sigourney Weaver as Ellen Ripley
Directed by Ridley Scott

    The film begins as the spaceship Nostromo is returning to Earth but encounters transmission
leading them to explore a nearby planetoid. An unknown alien parasite seems to attach to one
of her crew members causing distraught to the crew but eventually appears to fall off.

The facehugger (parasite) laid an egg into the victim and

eventually led to the alien being born out of the chest of him.

Click here to see the birth of the alien! (WARNING: GRUESOME)

At the time, the film's attention detail truly redefined horror by binding it
with science fiction.
The introduction of the facehugger and xenomorph (adult alien) were done
so meticulously to leave a lasting fear in viewers with realism

Link to IMDB page for more information

Aliens (1986)
Starring Sigourney Weaver as Ellen Ripley
Directed by James Cameron

    Following the end events of Alien(1979), Ellen Ripley is again
faced with the xenomorph after surviving her first encounter. With the
company she worked for being skeptical about her first encounters, she

queenloses her license and is declared mentally ill. An opportunity arises to
help a colony on the moon LV 426 that was the first planetoid introduced
in Alien that sent out a distress signal.

This film introduced a new component of the xenomorph life cycle, the Queen Alien. The Queen lays the eggs that produces the facehuggers first introduced in Alien.

The heroine, Ripley, further pushed a strong female lead in this popular film as one of the earliest
female action stars.

James Cameron picked up what Ridley Scott created and pushed for a more action
adventure approach to the franchise while keeping horror as an aspect crucial
to the franchise. This film also further the lore of the xenomorphs.

Link to IMDB page for more information

Click the image to watch an iconic scene of the Aliens film (WARNING SPOILERS)