Ah but there's more than 4 weeks to a month... 4 weeks is 28 days - months are either 30 or 31, not counting Feb (which just messes things up annoyingly).
If you do 30.5 (to split the difference and take in Feb's nasty 28 days) x 9 you get the number of days (around 274.5) then divide by 7 to get the number of weeks its 39.214... (or 40 for non-aspergers people unlike myself ). Which clears up the whole month/week conundrum tidily.
If you want to know what you are in months do your number of weeks multiplied by 7, add any extra days you're counting, divide by 30.5 and you'll get a rough idea of the number of months.
eg. I'm 21 weeks = 147 days
plus 4 days = 151 days
divided by 30.5 = 4.951 months
Which for the sake of not freaking people out, I'll describe as 5 months....
But this is always counting from the date of your last period. Conception is actually about 2 weeks later, so you do get 2 weeks "free" if you like (as an introductory offer). BUt since this is taken into account in both week and month calculation it doesn't make a difference in this case.
Using this formula I can categorically state, Dawn, that you are 4.721 months pregnant.
Mathematically speaking that should round down to 4 and a half months, but I'm sure you wouldn't get into too much trouble if you were feeling optimistic and said 5 - which you can legitimately (and mathematically) do tomorrow anyway.
There. Bet you wish you hadn't asked... And this makes me officially tragic.
DBx