From an essay by Peggy Orenstein in the April 21, 2002 New York Times Magazine:
[T]here is no word in English for a miscarried or aborted fetus. In japanese it is mizuko which is typically translated as "water child." Historically, Japanese Buddhists believed that existence flowed into a being slowly, like liquid. Children solidified only gradually over time and weren't considered to be fully in our world until they reached the age of 7. Similarly, leaving this world — returning to the primordial waters — was seen as a process that began at 60 with the celebration of a symbolic second birth. According to Paula K. R. Arai, author of "Women Living Zen" and one of several authorities I later turned to for help in understanding the [mizuko kuyo] ritual, the mizuko lies somewhere along the continuum, in that liminal space between life and death but belonging to neither. True to the Buddhist belief in reincarnation, it was expected (and still is today) that Jizo [a boddhisattva] would eventually help the mizuko find another pathway into being. "You're trying to send the mizuko off, wishing it well in the life that it will have to come," Arai says. "Because there's always a sense that it will live at another time."