"Within a day of receiving Hammond's memo, Powell wrote a private letter to Blackmun. "I am enthusiastic about your abortion opinions. They reflect impressive scholarship and analysis." But Powell quickly got to his real question, which was "whether you view your choice of `the first trimester' as essential to your decision." Powell noted how Blackmun himself had volunteered that this choice was "arbitrary" in the cover memo that had accompanied his new drafts, and voiced his own--or his and Hammond's--proposal: "I have wondered whether drawing the line at `viability'--if we conclude to designate a particular point of time--would not be more defensible in logic and biologically than perhaps any other single time." Quoting Judge Newman's language about the constitutional importance of fetal viability, Powell told Blackmun that "I rather agree with the view that the interest of the state is clearly identifiable, in a manner which would be generally understood, when the fetus becomes viable. At any point in time prior thereto, it is more difficult to justify a cutoff date." Powell observed that the Court did not have to say anything, and that Newman's opinion "pointed the way generally toward `viability' without making this an explicit ruling," but Powell's letter was the first intra-Court communication to put the option of extending constitutional protection for abortion choice all the way to fetal viability explicitly on the table."
January 1, 1970
https://en.wikiquote.org/wiki/Roe_v._Wade