also: はつご
noun
first child; firstborn
Refers to the first child born to a couple or family. Neutral term.
彼女は初子を去年出産した。
She gave birth to her first child last year.
初子の誕生を家族みんなで祝った。
The whole family celebrated the birth of the first child.
長子 is a more formal or legal term for the eldest child, often used in family registers or inheritance contexts, while 初子 emphasizes being the firstborn in a more everyday sense.
一人っ子 means an only child (no siblings), whereas 初子 simply means the first child, implying there may be later children.
Compound of 初 (first) and 子 (child). The reading ういご uses the older native Japanese reading うい for 初, while はつご uses the more common はつ reading.