@DiegoMadonna
I am always amazed during pregnancy discussions at the number of people who genuinely can't understand the concept of days/weeks/months.
As in, "9 x 4 is 36 so are pregnancies actually 10 months long?" Seriously?
I know. It's quite bizarre.
As someone said earlier in the thread, some people are most insistent that there are 4 weeks in every month. So as there are 52 weeks in the year, they think: 52 divided by 4 = 13, ergo there are 13 months in the year.
As has been said, how can people get to adulthood and actually believe a pregnancy lasts for ten to ten and a half months? Or that there are 13 months in the year? There are some really bizarre things that people think though - obvious things - that are utterly wrong.
Like I know some people who not only believe there are 13 months in the year, but also some people who actually think the world is flat, if you pull at face when the wind's blowing it will stick, if you dream you die in your sleep you will die for real, if you keep your farts in you will explode, all dogs are male and all cats are female, and if you swallow apple seeds, an apple tree will grow inside you.
So weird. 
It doesn't matter what the Romans used to believe, there are 12 months in the year, and there has been for 100's of years.