０ぜろ歳さい

also: れいさい

ゼ

ロ

さ

い

Meanings

under 1 year old; 0 years old

Refers to a baby who has not yet reached their first birthday. Often used in medical, developmental, or statistical contexts.

１いっ歳さい
１歳 means 1 year old, while ０歳 refers to a baby under 1 year old.
新しん生せい児じ
新生児 specifically means a newborn (first 28 days), whereas ０歳 covers the entire first year.

０歳
- ゼロさい
  Standard modern spelling using Arabic numeral and 歳.
- れいさい
０才
- ゼロさい
  Common simplified variant using 才 instead of 歳.
- れいさい
〇歳
- ゼロさい
  Formal or official style using the kanji numeral 〇.
- れいさい
零歳
rarely-used kanji form
- れいさい
  Rare kanji spelling; reading れいさい is less common than ゼロさい.
- ゼロさい
れいさい

Compound of ゼロ (zero) or れい (zero) and 歳/才 (years of age). The reading ゼロさい is more common in modern Japanese.