My Girlfriend's Child Vol. 1 by Mamoru Aoi

In this poignant and award-winning romantic shojo drama, a teenage couple's lives are changed forever after a positive pregnancy test.

Sachi and her boyfriend Takara are your average high school couple. They go to school together, hang out, and even engage in the more intimate side of dating. However, as time passes, Sachi starts to get the feeling that something isn't right and decides to buy a pregnancy test. Later, in the bathroom of a family restaurant far from her home, she sees the two red lines that will change both her and Takara's life forever.