I was pregnant 39+2 with my first and 40+5 with my second. Remember though that while the average pregnancy lasts 40 weeks, you're only actually pregnant for 38 of those because the first 2 weeks are counted from the first day of your last period, until the day of ovulation; before the egg has even fertilised.
Also remember that 4 weeks doesn't amount to one month; the average month is 4w+3d