also: れいさいじ
noun
child under one year old
Refers to an infant who has not yet reached their first birthday. Often used in medical, childcare, or statistical contexts.
ゼロ歳児の予防接種スケジュールを確認する。
Check the vaccination schedule for children under one year old.
この保育園ではゼロ歳児から預かっています。
This daycare accepts children from under one year old.
Compound of ゼロ (zero) + 歳 (age, years old) + 児 (child). The reading ゼロさいじ is a straightforward combination; the alternative reading れいさいじ uses the native Japanese reading of 零.