Japan and the 10-month Pregnancy

May 8th, 2006

Filed under: Comics, Medicine

Dawn e-mailed to ask about an editor’s footnote in the final volume of the manga Marmalade Boy which reads, “In Japan, the normal length of a full-term pregnancy is considered to be 10 months, not 9 months.”

That footnote is correct: Traditionally in Japan, pregnancies are considered ten months long.

Now, it’s not that Japanese women are pregnant a month longer than non-Japanese women, but instead it all comes down to semantics.

Quick medical background information: Pregnancies are dated from the last normal menstrual period and the average normal pregnancy is considered to be forty weeks long. An infant born at 37 to 42 weeks gestation is considered “full term.” An infant of less than 37 weeks gestation is considered “pre-term” (or more commonly “premature”). A pregnancy that lasts longer than 42 weeks is considered “post-dates.”

According to the Gregorian calendar, where months range from 28-31 days, this means the average pregnancy lasts a little over nine months.

On the other hand, if you’re counting the more traditional lunar months of exactly 28 days (i.e. 4 weeks), then 40 weeks = 10 months. The Japanese belief that a pregnancy lasts ten months refers back to their original calendar that used these shorter months.

So:

9 month American pregnancy = 9 calendar months = 10 lunar months = 10 month Japanese pregnancy

(And just to further confuse thing, the classical phrase regarding a pregnancy in Japan is totsuki tooka, which actually refers to a length of ten months plus ten days)

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9 Responses to “ Japan and the 10-month Pregnancy ”

  1. Aaron
    May 8th, 2006 at 2:43 pm

    I was taught that Americans traditionally date from the imaginary start date of the first missed period, which gives you 9 months, but in medicine we’d rather not use a date that never happened which gives 40 weeks.

  2. Melissa Gay
    May 8th, 2006 at 6:54 pm

    Hmmm. Interesting– if I’m reading you correctly, then my (American) OBGYN went by the Japanese pregnancy-dating.

  3. Melissa Gay
    May 8th, 2006 at 6:55 pm

    Oh, wait, I wasn’t reading it correctly. Note to self– never post a comment right before supper!

  4. Cindy
    May 8th, 2006 at 7:35 pm

    Not a comment for this specific post, but for the blog in general:
    http://news.bbc.co.uk/2/hi/health/4984042.stm
    the BBC stole your medical reviews idea! ;)

  5. Ralphy
    May 8th, 2006 at 7:55 pm

    Are you excited for XMen III?

    I think theres going to be a preview or something on Fox this Wednesday/Thursday.

    Im pretty excited for House tommorrow also.

    Later

  6. j.w chen
    May 8th, 2006 at 8:28 pm

    Some Asian countries in the Far-East did not incorporate the Gregorian calendar until late eighteen hundred (Chinese didn’t start until 1912). From what I know some traditional Chinese family would say ten months. Because of this, my fourth grade health teacher had to spend thirty minutes explaining to us why doctors bump it down to nine.

    Sorry, had a sudden urge of saying something that has little or no relevance.

  7. TORDELBACH
    May 9th, 2006 at 3:57 am

    Hmm, intriguing. Expecting our firstborn any minute now, I was rightly shocked all those long months ago to be told by our Doctor that a pregnancy was 40 weeks. As were the prospective grandparents, who you would have thought might have known better. So I can reveal that in Ireland at least a pregnancy appears to be dated from the first day of the last definite period - giving 40 weeks as opposed to the likely conception-date-to-delivery span of 38 weeks.

  8. John Nowak
    May 14th, 2006 at 10:46 am

    Interesting … isn’t it also traditional in Japan to bump the age of a person by one year? Basically, instead of being “one” because someone was “born over one year ago”, that same person is “two” because “they are in their second year.”

    It all comes down to whether you start counting at one or zero.

  9. Dawn
    May 15th, 2006 at 4:51 pm

    Wow! Thanks for such a thorough answer and the visual too.

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