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Lexical Leavings

by Tom Gally



Most Japanese-English and English-Japanese dictionaries, in their entries for 妊娠 and "pregnant," have examples like the following:
妊娠4か月である
She is in the fourth month of pregnancy. / She is four months pregnant.

She's six months pregnant.
彼女は妊娠 6 か月である.
I haven't yet found a dictionary that notes the subtle error of these translations. In Japan, pregnancies are counted in months of 28 days, and the normal term is said to be ten months. In English, calendar months are used and the normal term is nine months.

Here is what two encyclopedias say:
妊娠する前の月経(最終月経)の第1日から数えて分娩が何日目にあるかを調べた何万という例から、その平均がだいたい第280日目であることがわかり、これを出産予定日としている。この280日という数は40週に相当し、4週間を妊娠の1か月と考えれば、10か月になるわけで、しかも4週間は28日間で大多数の月経期間の日数とも一致するところから、便宜上、妊娠期間として採用されたわけである。(日本大百科全書)

The entire process from fertilization to birth takes an average of 266-70 days, or about nine months. (Encyclopedia Britannica)
(The difference in the number of days between 266-70 and 280 is not enough to account for the difference between nine and ten months.)
(February 19, 2003)
A correspondent sent the following comment:

The Japanese calculations are done from the date of the first day of the last menstrual period (LMP), whereas the Western calculations are done from the presumed date of ovulation, calculated by adding 14 to the first day of the LMP. That explains the discrepancy.
(March 14, 2007)