This is killing me. If it rings a bell, please help me. There's this scene in manga I've read (I believe here) where a young mother is going to take a bath with her baby, she fills the bathtub but then the phone rings so she tells him to please stay put and she goes for a few minutes and when she comes back the baby had gone inside the tub on his own and drown... Later she wanted to commit suicide.

Edit: I'm sorry for looking for something so depressing, I just remember I loved the art and plot, but I don't even remember if I read it here or if its even Yuri... The one above is pretty depressing too lol.

Gunjo? IIRC, she was a side character and it happens fairly early on in the series. The mother goes to answer a phone call invitation for her classmate reunion. Her husband's family blames her for her son's death and she tries (and I think succeeds) in drowning herself in the ocean.