In Theodore Dreiser's novel "Sister Carrie," Carrie can be seen as a fallen woman.
Given that Dreiser was a Naturalist, Dreiser's writings were based on the characteristics typical of the Naturalist movement. Naturalist writers believed that nature was stronger than mankind. Through this, mankind had no free will and could never overcome the powers of nature. (Nature was typically personified to show the supremacy of nature over mankind.) Based upon this, no matter what mankind, in this case Carrie, did, nature always had a plan for the character which would always overpower any plans the character had him/herself.
Carrie, therefore, is a fallen woman based upon the fact that she (through the beliefs of the Naturalist) will always fall in any circumstance in life. Regardless of the measures she takes to survive (leaving home, taking a job in the to help stabilize her financially, relationships which benefit her), she will assuredly fall. This is solely based upon the fact that nature is simply more powerful.