Based on their sex.

http://www.med.monash.edu.au/gendermed/sexandgender.html

> Sex refers to biological differences; chromosomes, hormonal profiles, internal and external sex organs.

> Gender describes the characteristics that a society or culture delineates as masculine or feminine.

May as well link some script from the Life of Brian to demonstrate that you can identify (gender) as a woman if you are a male but where's the foetus going to gestate (sex).

http://montypython.50webs.com/scripts/Life_of_Brian/8.htm