Is a babies age determined by date of conception or date of birth? What is the term used for determining the age? A Japanese friend told me that a pregnancy is 10 months. I have always used tanjoubi to mean a person's birthday, but is age actually determined by date of conception? Is a baby considered to be zerosai/marusai/reisai at birth, or older?


In the past, Japanese ages used to be counted similarly to Korean ages, as ajsmart's comment talks about, but Japanese ages have been "westernized" for a long time now.

My Japanese friends who have started families always refer to their children's age from birth. I have heard newborn babies' ages described as [0歳]{ゼロさい} "zerosai", and also [1ヶ月]{いっかげつ} "ikkagetsu", and so on for months, [1週]{いっしゅう} "isshuu", and so on for number of weeks, and if you want to specify down to the day for some reason, you add …と[1日]{いちにち} "to ichinichi" etc for days.

So a baby that is 3 months old would be [3ヶ月]{さんかげつ} "sankagetsu".

A baby that you want to describe as (for some strange reason) 2 years, 4 months and 5 days old would be [2歳]{にさい}[4ヶ月]{よんかげつ}と[5日]{いつか} "ni sai, yonkagetsu to itsuka".

  • 1
    What is to ikka? Maybe you tried to type "to mikka" (と3日)?
    – Chocolate
    Jul 20 '18 at 0:07
  • 2
    「ひとすか」て何ですねん・・・ 「1日」は「いちにち」でよろしいですやん・・
    – Chocolate
    Jul 20 '18 at 3:34

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