Eraserhead
Thank you David for giving us the huge space of interpretation so everyone can interpret this differently.
My thought is that the baby is shown as an Alien is because it's unwanted, especially by the mother - which is why she had those worms under the blanket in the scene were they lay in bed. She doesnt like the role of a mother and views the worms as aliens from her uterus. I feel like this movie is a representation of exactly this and how hard it is to be a mother/mother figure to some women or atleast the parts people dont talk about enough, yer the ending had me quite confused.
There is a lot more to interpret but that is one message or atleast my personal interpretation of the main part of the movie after a first time watch.