When does sailor chibi moon find out Serena is her mom?

Sailor Chibi Moon discovers that Serena (Usagi Tsukino) is her mother in the "Sailor Moon S" arc of the anime. This revelation comes during a pivotal moment when she learns about her true identity as Chibiusa, the daughter of Usagi and Mamoru from the future. The emotional impact of this discovery deepens their relationship and adds complexity to Chibiusa's character development throughout the series.