Taanis Bechorim

Generally the minhag was that only the boys fasted (there is no Kedushas bechorah for the girls, although in the past there were single girls who did fast). R' Yozfeh Shamesh mentions specifically that girls do not fast (but the Noheg Katzon Yosef brings that there may be an "inyan" for girls to fast too).
The only case that there is no question that a woman fasts, is when the father is a bechor, and a boy is born, and then the mother fasts for her son until he's bar mitzvah.

Michael FRBSH

See Shi'ur (in Hebrew) about Ta'anis Bechorim by Harav Gans Shlito in this link .