Wachenheim, 44, died. But baby Keston Bacharach survived, his fall cushioned by his mother's body, with only a few scratches.
New information reveals the motivation -- guilt and fear -- that led to Wachenheim's fatal decision to end her own life and try to end her son's.
According to a law enforcement official who spoke with the New York Times, Wachenheim left a 13-page suicide note explaining the guilt she felt over two previous incidents in which her child had fallen.
Wachenheim wrote of her belief that the "shameful incidents" -- one, when Keston had fallen from a play set onto a wooden floor, and another, when he had rolled off a bed -- were the fault of a series of seizures and concussions that would cause Keston suffering his whole life.
She wrote about how no one would believe her, but that she had seen the changes in her child. He cried more often, she said, and slept more. The guilt over the accidents had sent her into a spiral of depression, she said, unable to socialize.
She also said what she was about to do -- try to kill her son along with herself -- was "evil."
Wachenheim was a court attorney in the Manhattan State Supreme Court system, but had been on leave to take care of her new baby. She and her husband, Hal Bacharach, lived in an upscale apartment building in Harlem.
The New York State Department of Health says that, while as many as 20 percent of new mothers suffer from postpartum depression, only a tiny fraction of moms -- just one or two in a thousand -- experience a condition called postpartum psychosis, which can cause suicidal or homicidal thoughts.
Notable deaths of 2014 [PHOTOS]