Landing on her back. From the New York Daily News:
A Manhattan lawyer strapped her 10-month-old son to her chest and leaped to her death from an eighth-story window in Harlem on Wednesday—and the baby miraculously survived with little more than scratches, police sources said. Cynthia Wachenheim, 44, who left a suicide note castigating herself for being a bad mother, landed on her back after taking the fatal plunge. Baby Keston was in a harness. He bounced off his mom on impact and then rolled away from her motionless arms.
Pertinent details: Wachenheim was believed to have been suffering from postpartum depression. Also: "Wachenheim was convinced her baby had cerebral palsy, although doctors said there was nothing wrong with him."
Full tragic story here.