。
She gave birth to her first child on Monday.
Literal
She [topic-は] Monday [on-に] first-child [object-を] gave-birth-to.
Two everyday building blocks: the time particle に on 月曜日 (the standard way to anchor an event to a specific day, week, hour, etc.), and the kanji prefix 第~ to form ordinals. 第一子 'first child,' 第二子 'second child' — same prefix as 第一次世界大戦 ('First World War') or 第三章 ('Chapter Three'). 産む is the everyday verb for 'give birth,' more conversational than 出産する.