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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:
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:

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)