I get NIH press releases at work - and they have a podcast for healthy pregnancy. I didn't listen to it, but in scanning the tips (hey, I definitely want to put them to use someday!) I saw this: "A normal pregnancy lasts 40 weeks, Dr. Spong said. Doctors consider delivery before 37 weeks to be preterm."
SO - 40 weeks/4 weeks per month = 10 months.
(Even pre-term - 37 weeks/4 weeks per month is 9.25 months!)