With a baby bump protruding from beneath her white jacket, astronaut Naoko Yamazaki announced Wednesday she was retiring to concentrate on caring for her second child, due in October.

However, Yamazaki, 40, said she hoped to eventually be engaged in space education at some point in the future.

"After weighing up whether I should train for another decade to go into space again or take a new path, I decided to retire [and do the latter]," Yamazaki said at a press conference in Tokyo.

Despite retiring from the Japan Aerospace Exploration Agency, Yamazaki said she had not turned her back on the agency.

"I want to support JAXA from the outside and help the public feel closer to space exploration," she said.

Training an astronaut costs almost 300 million yen (S$4.7 million) a year. JAXA astronauts usually use their experiences in space to instruct space-exploration trainees and others, so Yamazaki's early retirement is somewhat unusual.

Yamazaki, the nation's second female astronaut, said she had not made any concrete long-term plans. "I just want to spend more time with my family for now," she said.

Yamazaki was one of the seven members aboard space shuttle Discovery when it flew to the International Space Station in April 2010. She was the last Japanese astronaut to fly on a space shuttle.