OK, answered my own question. Apparently, it was the decision of the Supreme Court in the case of R. v. Morgentaler [1988] 1 S.C.R. 30. The Court ruled 5 to 2 that section 251 of the Criminal Code was an infringement of Section 7 of the Canadian Charter of Rights and Freedoms, which states:
7. Everyone has the right to life, liberty and security of the person and the right not to be deprived thereof except in accordance with the principles of fundamental justice.
I am an infant in such matters. I don’t know if any of the justices relied on the Criminal Code’s definition of human beings in order to exempt certain children (the Code’s word, not mine) from section 7 of the Charter.