A bio-girl is a term used to refer to a non-trans female. A bio-girl is naturally born as a woman and is psychologically female. Her gender is also socially female. In short, a bio-girl is born with female anatomy and self-identifies as female.
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Bio girl is just one term used to differentiate genetic females from those who have transitioned. As a result, this term is controversial in some circles because it excludes trans women from full womanhood. Many trans women argue that biology is not the defining characteristic of womanhood.