Karl M. Baer (20 May 1885 in Arolsen, Germany – 26 June 1956 in Bat Yam, Israel) was a German-Israeli author, social worker, reformer, suffragette and Zionist. Born female and named Martha Baer he became, in December 1906, one of the first people to undergo sex-change surgery, and one of the first, in January 1907, to gain full legal recognition of his gender identity and to have a new birth certificate issued reflecting his new gender. Baer wrote notes for sexologist Magnus Hirschfeld on his experiences growing up as a girl while feeling inside that he was male. Together they developed these notes into the semi-fictional, semi-autobiographical Aus eines Mannes Mädchenjahren (Man's Years as a Young Girl) (1907) which was published under the pseudonym N.O. Body. Baer also gained the right t