Not all mommies can conceive without some help. Thank goodness for in vitro fertilization! Alien test tube babies have to wait for just the right moment to move to facehugger stage. Once there, humans like Carter Burke (Paul Reiser) can help them move into chestburster stage.

Aliens, William Hope, Lance Henriksen, Paul Reiser, Michael Biehn, 1986. TM and © 20th Century Fox. All rights reserved. Photo courtesy of the Everett Collection.