The physical structure of one’s reproductive organs that is used to assign sex at birth. Biological sex is determined by chromosomes (XX for females; XY for males); hormones (estrogen/progesterone for females, testosterone for males); and internal and external genitalia (vulva, clitoris, vagina for assigned females, penis and testicles for assigned males). Given the potential variation in all of these, biological sex must be seen as a spectrum or range of possibilities rather than a binary set of two options.
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REFERENCE: Gender Spectrum Guide to Gender Terminology