Emi Wakabayashi, 34, who lives with her father, younger brother and first son, was initially arrested after police discovered the corpse of a newborn infant near their home in rural Ehime prefecture in western Japan.
The skeletal remains of four more infants were found in sealed plastic bags near her home yesterday, a police official told AFP.
Officers began investigating after being told that Wakabayashi had appeared for several months to be pregnant. But she did not have a baby with her when a police official attended a community meeting, he said.
"She hinted that she was involved in the killing of some of the babies," he said, adding officers were also questioning her family and were carrying out DNA tests to determine the parentage of each child.