Because I get into this debate all the time! I mean not GET INTO it, just realize that people are sort of confused about the whole "months" thing.
Most everyone knows that a pregnancy lasts about nine months.
The medical world and actual pregnant people like to keep track in weeks because it's more accurate. The average pregnancy lasts 40 weeks, counting from the date of a woman's last menstrual period (LMP). So pregnant women are constantly faced with stating their "progress" in terms of weeks for the scientific/experienced world, months for the normal people.
And that's where the bad counting starts. I got involved in the debate way more than I wanted to because I made this script to tell me, in very simple terms, how pregnant I am: http://spacefem.com/pregnant.php Because let's face it... when you first take that little pee stick test you're obsessed with the "progress" number, but as time goes on the weekly count gets pretty blurry. I'm currently... in the 20s someplace.
Other people in

- "I'm in my sixth month, so I'm six months pregnant." Like our grocery store checker tonight. People NOOO! In your first year of life are you one year old? You're six months pregnant when you've completed six months.
- "There are four weeks in a month." Again, close but wrong. I know it's nice to think that at 16 weeks you're four months pregnant... but since all months have a teeny bit more than four weeks in them it doesn't work out. Four weeks is 28 days, most months are 30-31. Even if every month was a nice even 30 days, four months would be 120 days and 120/7 is 17.14 weeks... at 16 weeks you've got some time to go, kids. And hell, has anyone noticed that if there were four weeks in a month pregnancy would last ten months? How will you explain that your December pregnancy will end in September?
- "I found this random chart that says 21 weeks is five months so it's TRUE!" Close... but since weeks don't correspond exactly with months, you can't have a chart that says exactly which week corresponds with what month. Unless you do a lot of sloppy rounding. And I am not working sloppy rounding into a perfectly good script. I also don't care what your random charts say if they're wrong.
So me, my script, and my big mouth have gotten me into all sorts of trouble because I insist that this is the only way to convert weeks to months:
[Weeks you are at] * (12 months/365.24 days) * (7 days in a week) = Months you are at
And you have to round down, if you're going to round, because you're trying to get to "months you've accomplished". And of course calendar dating is also acceptable... if your last period was March 12, and March is the third month of the year, you'll be one month on April 12th. Two months on May 12th. Three months on June 12.
I refuse to believe that "pregnancy months" are different or special, I refuse to round up because some inaccurate chart says you can. Please people, pregnancy is important. You're bringing a new life into the world. The least you can do is check the math.