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She was born in 1947 no specific date was listed, although it says Jan 1. 1947 this isn't a confirmed date.
Although not seen in the manga, in the book that the manga was based on, Dr. Nagao before murdering her, attempted to rape her when he discovered Sadako to have Testicular Feminization Syndrome. Although Sadako has the appearance of a beautiful young woman, "she" has chromosomes XY (Male).
In the a version of Ringu 0: Birthday it introduces the concept of 2 'Sadakos' which wasn't introduced in the earlier films/books/manga. This implied that Sadako in her youth split in to 2 identical girls. One being "normal" and the other one being more violent and sadistic.