noun
menarche; first menstruation
Medical and everyday term for a girl's first menstrual period.
初潮を迎える平均年齢は12歳前後です。
The average age for menarche is around 12 years old.
娘に初潮が来たとき、母は優しく説明し
When her daughter had her first period, the mother explained it gently.