noun
firstborn; first child; eldest child
Refers to the first child born to a parent or couple. Neutral term used in family contexts, medical records, and official forms.
彼女は第一子を出産したばかりだ。
She just gave birth to her first child.
第一子が生まれたとき、夫はとても喜んだ。
When their first child was born, the husband was overjoyed.
Compound of 第一 (first) + 子 (child). The reading だいいっし follows the standard on'yomi with a geminate consonant (促音) between いち and し.