From Aerandria Scans:
One day, Mizuho, a university student, receive news that her classmate in high school, Origuchi Haruka, passed away. Mizuho doesn't remember being best friends with Origuchi, but apparently, Origuchi told her mother that Mizuho was her best friend. And now, her mother wants Mizuho to help her find the boy Origuchi dated during high school. Mizuho told Origuchi's mother that Origuchi was a very plain girl, and she was never seen with a guy, but her mother told Mizuho that Origuchi got pregnant and had an abortion when she was in high school. Mizuho was shocked, but she still starts asking her other classmates in high school and following the traces Origuchi left behind.
Note: Won the 58th Shogakukan Manga Award in the Shoujo manga category in 2013.