Naisho no Tsubomi

 is a Japanese slice of life romance "shōjo" manga series authored by Yū Yabūchi, and published by Shogakukan. 8 volumes were released in Japan, the last on April 27, 2012. An OVA anime for the series was released on April 24, 2008. A second OVA episode was released in June, 2008. The series received the 2009 Shogakukan Manga Award for children's manga. It was published in French by Delcourt. Sentai Filmworks released the OVA in North America in January 2018.
Plot.
"Naisho no Tsubomi" tells the story of Tsubomi Tachibana, a fifth grader dealing with rather sensitive issues such as her mother's pregnancy, her first period and the strange feelings she has started to get when around boys. When she first met Daiki, she tends to blush a lot when she is around him, she does not know what that feeling is at first, but further into the story she then discovers that she, in fact, has fallen in love with Daiki.
Each volume tells the story of a different girl, always with the given name of Tsubomi and similar appearance. In each case the story deals with various issues the girl experiences with early adolescence, including physical matters such as menstruation and breast development, personal issues such as the first crush, and more uncommon but dangerous issues such as the risk of predation (this was not included in the anime). Each story also includes some source of candid advice and information to support the girl, such as the spirit of a future sibling (volume 1), an alien posing as a twin brother (volume 2), or even another self from a parallel universe (volume 5).
Reception.
On manga-news.com, the staff gave it a grade of 14.63 out of 20.