I was talking to Manami the other day and she mentioned that she was in her seventh month of pregnancy, and so I said, “Wait, aren’t you due in October?” She said yes, so this was her seventh month.  I was very confused, and so was she.  However, we eventually figured out that in Japan pregnancies are said to take 10 months, while in the US pregnancies are said to be 9 months.

Why are they different? It depends on how you count the time.  A pregnancy for humans lasts about 40 weeks, or 280 days.  If you think that a month lasts 28 days then that is exactly 10 months. However, if you think that a month lasts 30 or 31 days (and 28 for February), as they do on the calendar, then a pregnancy would last 9.2 months (280 total days of pregnancy ÷ 30.42 average days per month). A lot of people in the US consider a month to last 4.5 weeks, which would be 40.5 weeks of pregnancy over 9 months.

Both systems are correct, it just depends on your definition of what ‘a month’ is.  Something to keep in mind as you travel out of the country.

Here is a good video in both English and Japanese that talks about the difference (1:12):