"Many, perhaps, will think it idle to go farther in demonstrating the immorality of large families, but since there is still an abundance of proof at hand, it may be offered for the sake of those who find difficulty in adjusting old-fashioned ideas to the facts. The most merciful thing that the large family does to one of its infant members is to kill it. The same factors which create the terrible infant mortality rate, and which swell the death rate of children between the ages of one and five, operate even more extensively to lower the health rate of the surviving members."
https://wwnorton.com/college/history/america-essential-learning/docs/MSanger-Woman_and_New_Race-1920.pdf
And here's a link to a Planned Parenthood "Fact Sheet" falsely claiming that "Sanger was making an ironic comment — not a prescriptive one" when the full quote and the paragraphs around it don't suggest an irony whatsoever.
https://www.plannedparenthood.org/files/8013/9611/6937/Opposition_Claims_About_Margaret_Sanger.pdf
That is encouraging—so the only blacks Sanger wanted to kill were infants in large families. :-(